If I create a multiclass character with classes of two different XP categories -- let's say a monk/psion -- is it better to take levels in the cheaper class first, so as to level up quickly and increase survivability? Or is it better to take levels in the more costly class first, when XP requirements are lower, so that advancement isn't slowed to a crawl later on?

Also, is the cost of gaining a level based on your current class, or the class you take when you level up?

I will assume you are talking about Wizardry 8.

Multi-classing in Wizardry 8 has always been a bit iffy. The issue is that there is not that much actual overlay.

The only way I've ever heard of it being really done well is if you start as a pure mage, work them up to where you can comfortably cast top spells and then for the last level or two before the end game go to a class that can still cast those spells so you can get better defense. Of course the issue it that you lose the bonus to the class skill, psionics in this case. But that is not always a big issue as often times it does not really do anything once it goes above 100%.

Anyone else? When I asked about the best strategy for multiclassing, I was really hoping to get an answer other than "Don't."

Hello,
Here are a few other discussions on this topic along with some strategies for it.

One strategy is to take one level of rogue and then powergame to get your stealth to 100.

https://steamcommunity.com/app/245450/discussions/0/846964801585753151/

http://www.zimlab.com/wizardry/recovered/avenstar/dclass.html

https://www.reddit.com/r/wizardry8/comments/5ln8on/how_does_multiclassing_work/

https://gamefaqs.gamespot.com/boards/374906-wizardry-8/43043702

Grinding Stealth up to 100 is not necessary nor pleasant, in fact last 10-20 points to reach 100 are the worst and it's better to skip them. Free 6-8 AC points are nice but 2 more are not worth that hassle...

Start with either Rogue, Monk or Ninja, raise their exclusive skills (mostly Stealth but also Criticals and maybe Martial Arts) then reach 2nd level as some magic class, preferably Bishop which can use any spell school.

The most optimum strategy? Pick your class, don't level up so you can raise your skills (especially spell schools) on low level monsters.
That's because you can use class-exclusive skills but can't raise them up when you change your class.
Monster spawns at certain levels (depend on the zone), which in your case mean all levels from each class combined.

if you want to use a polarm then go Monk first because Psion can't train in the skill but the best one she can use gives +20 so then swap to Psion is order to be immune to mind spells for the late game when those spells become an issue... the fact a Monk levels slower is off set by the fact that early levels come cheep

if using a staff or just want to rock with spells then of course Psion first but they are more crowd control than actual damage with their best spell not needing targets and just hitting every bad guy within X of the party

Wizardry 8 does not have 1 correct answer but there are lots of wrong ones ;)

Here's one example where multiclassing can give you an expert skill earlier::

Build 1:
* Dwarf Lord (this example also works with Valkyrie)
* At character creation, put 9 points into STR for a total of 64. (This is the max for 25 starting bonus points.)
* 3 STR per level.
* At level 13, STR will reach 100 and Power Strike will be learned.

Build 2:
* Dwarf Priest
* At character creation, put 15 points into each of STR, DEX, and SPD. (50 starting bonus points gives you the possibility of putting 17 in a single stat.) You now have 70 STR, 50 DEX, and 50 SPD.
* I recommend starting with Mace & Flail > Staff & Wand. This will give you a mace as starting equipment, allowing you to start building Mace & Flail early (since the best offhand weapon is in this category) (assuming you're doing Lord and not Valkyrie).
* At level 2, change to Lord, as you qualify for it. Put 3 points into STR.
* With 3 STR per level, you get Power Strike at level 11. 2 levels sooner than with the previous build.

Drawbacks of Build 2 do exist, however:
* Lower HP, somewhere around 1 Lord level's worth. (Specifically, you get 2 levels of Priest instead of Lord growth.)
* Lower attack rating, equivalent to 1/2 a Lord level.
* Possibly 1 level's worth of HP regen behind.
* At level 5, you still can't boost magic skills.

But you do get a head start on some skills (notably Divinity and Divine Magic, and 1 more spell pick than you would otherwise have.

For an early/late game trade-off, favoring early game, you could spend more time as Priest to get some other spells, like Identify Item and Armorplate, if your party needs them, at the expense of long-term fighting ability.

I was wondering the same thing. In my thread, I created a W8 Magic Damage Party. All Humans: Bard, Gadgeteer, Priest, Alchemist, Psionic, Mage. Humans end up with the best total stats in the end, at the cost of learning expert skills a few levels later. Once I get my pure casters to level 24, which is the level needed to cast level 7 spells at PL7, and assuming all my realm skills and spellbook skills are at 100, I don't see why I can't change them over to their respective hybrids that use that spellbook, or maybe having a full party of fighters with maximum magic abilities. I know that I would lose the 25% bonus to their spellbook, but 100 Wizardry/Divinity/Psionics/Alchemy is still pretty good.

Thought of an interesting build:
* Race: Human (when multiclassing with Bishop, we need all the stat points we can get, and having them spread out is a feature rather than a bug)
* Starting class: Fighter (55 STR, high HP at start, and 40 bonus points to allow easily meeting Bishop requirements)

The strategy is as follows:
* Consider creating a Priest in that slot, with Mace & Flail > Staff & Wand. This allows us to start with a mace.
* Then create the Human Fighter. Raise INT, PIE, DEX, and SEN to to 55, except leave one at 53, leaving 6 bonus points. (DEX is at 50 due to Fighter requirements, so we only need 5 points here, not 10 as for the other attributes), and STR at 61 (so Power Strike at level 14; late but not horrible, particularly in comparison to hybrids)
* Mace & Flail will be our main weapon skill; the Bishop gets it, and it's also good if we want to dual wield late game (in which case, put points into that skill now).
* Level 2, change to Bishop. Continue boosting fighting stats. (Note that bonus points gained at level up can be used to qualify for another class.)
* Focus on Wizardry and Divinity as a Bishop; you can ignore Alchemy and Psionics for this build.
* As a Bishop, learn the important non-combat spells, like Enchanted Blade, Armorplate, Knock, and X-Ray. Also, aim to learn the portal spells at level 12. Other spells worth learning include healing and some status cures (most notably Lesser Condition and Paralysis)
* After reaching level 12, practice until the utility spells can be used reasonably reliably at PL7 (except for X-Ray, which doesn't need a high power level), as you won't be able to later.
* At level 13, change back to Fighter, and enjoy having a fighter (albeit one with fewer HP and worse attack rating) who can cover all the needed utility spells, and has a portal as well.

This can be useful if you're having trouble finding room to cover the utility spells, but still want a melee focused character. Fighting ability will eventually, around endgame, catch up to Bard/Gadgeteer, except that this character can Berserk and use Fighter equipment.

Now, next I'm going to try researching the possibility of using Lord instead of Fighter in this sort of build (for the Dual Wield bonus and the ability to continue developing Priest spells late game).

Lord version:
* To multi-class to Bishop at level 2, you *must* put 7 points into each of INT and SEN, and at least 5 into DEX, leaving only 1 more point to distribute. This means Power Strike won't appear until level 17 (level 2 is spent solely meeting the class change requirements).
* Alternatively, you can delay Bishop levels in order to get Power Strike sooner.
* 9 levels of Bishop is sufficient to reliably cast Missile Shield and Enchanted Blade at PL7, and power level isn't as important for X-Ray. (Plus, if you have a Gadgeteer, you eventually won't need this character to cover it. Note, however, that this means not getting the portal spells until level 15 at the earliest, and your magic skills will not improve from 11-14.
* One other way to get Power Strike sooner is to hold off on creating the character until you're about to use the Trynton Fountain, but then you miss out on early skill training, so this may not be the best choice (and this build requires more XP than anything other than pure (or late-game) Bishop/Ninja).
* Valkyrie works out the same if you prioiritise STR; if you don't, you have a bit more freedom, as the STR requirement for Valkyrie is lower, giving you an extra 5 bonus points.

so you start as a Priest which gets a mace equipment at the start and then remove that character to replace with a Fighter to begin [i.e, keeping the mace]... why would you not want the Priest 2 spells at the start?

p.s, start with an imported party to get 4 levels heads up and good gear

Because I'd rather have more HP at the start. The extra spell for starting as a Priest can be replaced with a spellbook later; the extra HP for the Fighter level can't be.

Also, a level as Priest means one fewer level as Fighter (missing half a level's worth of attack rating), or one fewer level of Bishop (meaning one fewer caster level of non-Priest spells).

There's also stat requirements. To work Priest into the build, I would need 5 more points of Piety (Bishop requires 55, Priest 60) and 5 more points of Vitality (Priest requires 55, while Fighter only requires 50).

By the way, I found another interesting possibility. If you want both Bishop levels and the Giant Sword on the same character, you can start the character as a Mook Valkyrie, change to Bishop at level 2 (you'll meet the stat requirements), and then change back to Valkyrie later. This set-up actually does meet the Bishop requirements with just the race and Valkyrie minimums, and I'm pretty sure you can class change the level that you pay off the rest of the character's bonus point debt (you can't before then, so doing this with Lord means no Bishop for another level). The drawback is that this character isn't in a good position to get expert skills (Power Strike is something like level 19 if you aim for that skill first).

At the cost of (unless you throw away the extra 4 levels):
* The ability to distribute starting bonus points. (This makes expert skills come later, which is an issue particularly for offensive casters, as the game seems to be balanced with the expectation that they would have Power Cast in the teens.)
* The ability to be a pure Gadgeteer. (Apparently not possible even with a hacked save file; characters in glitch classes apparently import as Fighters.)

Another early expert skill setup:
* Start as a Faerie Mage. Put INT at 77, PIE at 52, SEN at 55. This will leave 6 points leftover.
* At level 2, change to Bishop. You'll only have 3 spare bonus points this level (need 55 PIE for Bishop), but you can still put them in INT.
* Use the Trynton fountain, while still boosting INT at level up.
* At level 7, you get Power Cast, and (unlike a pure Mage) you can use it with non-Mage spells (particularly Priest spells like Heal Wounds and (at level 9) Superman).
* Snakespeed can be obtained at level 14. (64-66 at level 1, no increase at level 2, and 12 levels of 3 points per level to max it out.)

Drawbacks:
* Will be one level behind for non-Mage spells compared to a pure Bishop.
* Faerie race is used here with all the drawbacks it entails. This set-up can work with Elf, though Power Cast is at level 9 here, I believe.
* HP is lower than that of a Bishop I believe the difference is something like 4 HP at 45 VIT. (Note that this difference does not grow with level, and the character still gets more HP than a Mage.) The Faerie's low VIT doesn't help matters here.
* Still have the slow Bishop XP table, and no primary skill bonus to Wizardry. (You could choose to abandon Bishop once you have all the non-Mage spells you want at good enough power levels, which will get rid of this drawback, but will make the character's non-Mage spells and HP fall further behind.)
* Faerie can't use the Robe of Rejuvenation or the Snakeskin Boots, so this build isn't as good in the long run.

Variations:
* Put fewer points in INT, and more points in SPD. Power Cast will come later, but Snake Speed will come earlier.
* At level 2, boost SPD instead of INT; this has the same effect as the previous option.
* Level 9 Snake Speed (on a caster!) may be achievable, but this will push Power Cast back to level 12, so you'll need to make a choice.
* Boost something other than SPD, if you'd rather have more HP or something else.

What about starting off as a Bishop, then changing over to a Fighter? You would learn all of your important spells by level 18, then have someone good at fighting(you probably will want to train with Mace and Flail since Bishops can learn that skill.) The only problem is that you won't be able to cast level 7 spells at PL7 reliably until level 24.

Problem is, when to change to fighter, as if you want to cast your high level spells at high power levels, you'll need a very high Bishop level.

Also, remember that Fighter levels don't count for getting past enemy spell resistance, so the character will have serious difficulty against higher level enemies.

(Worth noting that you're also stuck with Bishop HP for the first level, which counts twice.)