Literal: The giant spider you stabbed to death dropped a literal pouch of 30 gold coins.

Inferred value: The 30 gold isn't literal, but rather the exchange value of what parts of the spider could be sold in exchange.

Barter Goods: The spider drops venom glands, which you can trade for gold. (They also have additional uses.)

Inexplicable goods: The spider has dropped a wand, which none of your characters can use, but it can be sold for 30 gold.

Exceptionally dishonorable mention to: Lunar Dragon Song and it's special "Fuck You" system; you can get item or experience, but not both at the same time.

So, how do you like it? Any interesting systems I overlooked?

A creature just literally dropping gold on death makes no sense. A creature having gold and other valuables strewn about its lair, from past adventurers it's killed, that makes sense.
Inferred Value, or Barter Goods, is my preferred way of handling this.
The additional use case in Barter Goods also ties in with Inexplicable Goods. What you can't use now, you might be able to learn how to use, later.
Gamers should never be forced to choose between XP or physical loot, on killing enemy critters in a single player CRPG. When it's a multi-player party in a CRPG, that's a very different case. Then, it's up to the party to decide how the physical loot will be divided up.

There are MANY times in RPGs that I'm saying "If only I could buy skill points, attribute points, etc. with the millions of gold pieces I have unspent"! These days I'm into Titan Quest. I'm about halfway the main game and I've got 4,5 millions gold. I rarely buy anything, since most good and rare pieces (blue items) come from loot and not from merchants. I wish I could spend my gold to buy attribute points instead!!

I like the simple approach where you directly get money from enemies, rather than getting items that you have to sell for money. Makes inventory management a lot simpler, and means less work when you do finally get to the store.

One interesting system I overlooked happens in False Skies, once you get to the later part of the game. At this point, a new currency is introduced, that has something like different denominations of bills, except that you can't convert between them. As a result, buying cheap consumables and restoring your party doesn't affect how much high-end equipment you can buy, and vice versa. I think it's a rather interesting system. It may not make sense, but it's simple to implement, and has lower cognitive load on the player than the other alternatives, and there's something to be said for that. What about a system where enemies always give money, and you can use the money either for items or for powering up your characters?

SaGa 1 does this, at least if you're using humans. SaGa 3 (original, not remake) does this for robots.

I think I may have seen this sort of mechanic in some non-RPGs as well. Maybe Romance of the Three Kingdoms (NES)? And I remember a sports game where you could improve a player, but it had the side effect of increasing their salary. (Incidentally, Might & Magic 2 is an RPG that does something like that; if you level up a mercenary, their daily cost increases, though you don't have to pay an upfront training cost as you would with player-created characters.) Every now and then, there's an RPG that actually lets you do this, like the aforementioned SaGa 1 and 3 (though, in SaGa 1, it gets to the point where spending the money on stat boosts takes far more time than earning the money in the first place). I think some FIre Emblem games let you do this, and although it isn't an RPG (by my definition), Ys 6 allows you to buy HP increasing seeds late game.

Also, Avernum 1 and 2 have a shop somewhere that sells an item that permanently grants skill points. If you buy out the entire shop (expensive!), the shop will restock, complete with more of that item. (Note: This doesn't apply to the later remakes, but may apply to Exile 1 and 2. Also, Avernum 3 instead gives you a recipe for creating that item, though that recipe is treated as a major artifact, making it as hard to get as certain endgame pieces of equipment.)

That rarely works, because usually the spider will drop both gold and parts you can harvest and sell.

Honestly the spider dropping literal gold doesn't bother me much, though that's mostly because I'm just used to the cliche, and the fact.. well, those are game, a lot has to be "gamified" to be fun. A lot depends also on the particular game, the overall mechanics and tone it's going for. For example I actually like the way Bard's Tale (the 2004 one) did it, where items inferior to the ones you're already using were immediately exchanged for gold upon picking them up. But it's definitely not a system that would work in every game.

What does bother me, and that's just me being silly, because compared to the spider carrying around a wallet it's small potatoes, is the fact most RPGs default to using gold coins as basic currency. Like it's normal for every Tom, Dick and Harry in a medieval world to run around with gold in their puch, while actually most lousy bastards would be lucky to see a silver penny. Inflation in those magical kingdoms must be a bitch if a portion of measly stew will cost me a couple gold coins.

Merchants in Titan Quest are some of the best in any aRPG because they can offer up to rare items. So much so, that it's actually valid to farm them occasionally. They're the best source of rare (green jewelry) with skill bonuses and good rings in general, Hallowed helmets with +1 to all skills, Staves with skill bonuses etc.. And items like these can cost tens of millions a pop. If you think 4.5 million is enough, you are in for a rude awakening :) I even had to specifically farm gold a couple times because I kept running out. In 99% other games, merchants become pretty much irrelevant after a couple hours.

How do you feel about the Morraffware games, where your currency of choice is jewels?

Basically, they're acting as a money sink.

There's some other money sinks that I've seen. For example:
* Expensive consumables. This includes elixirs in some final fantasy games (2, 4, and 5, though i n pre-GBA FF2 limited inventory space keeps this from being effective as a money sink), shurikens in FF3, and others
* Permanent stat boost items, as one poster suggested (see the Avernum 1/2 and Ys 6 examples I mentioned; note that the SaGa 1/3 examples are not money sinks)
* Similarly to the above, minor upgrades to endgame equipment could also qualify
* Items needed for side quests (see FF7's chocobo breeding, as well as a similar mechanic in Crystal Project; Crystal Project also lets you buy items you didn't steal from bosses, but they're expensive especially if it's a late game boss)

Final Fantasy 1, incidentally, has no money sink, and it's not unusual to max out your money before completing the game. Dragon Quest games generally don't need money sinks, as equipment prices usually outpace the money you get from enemies, but if you need to you can spend excess money on casino coins.

This was the case in Ultima 9, where there was only a 4-digit number for your gold. So, you could reach 9999 gold way before the end of the game.
I also remember that in Dungeon Siege you could max out your gold (I don't remember the max number there) some time before the end. Any extra gold was dropping on the ground beside the merchant you were selling loot to!

Yeah, at low levels, the merchants are not really worth spending too much time on. Items can't even spawn with the good affixes yet. For the +2 to All Skills amulet, you need a merchant in an area level around 50, IIRC. But you should already be able to hunt for a Hallowed helmet (+1 All Skills). Merchant levels increase as you progress through the game/difficulties.

The merchant farming is only really worth it in late Epic or even later, at least act 3. Also make sure to farm the merchants in areas that have separate merchants for warrior/mage goods, not the general goods merchants. Locations like Elysium or Rhodes are great.

Best way to farm gold is picking up all yellow/green items in the late game and selling them. The gold cap you get per item increases the later you are in the game and sale prices skyrocket as you progress. You can get 250K gold per item in late Legendary. So again, farming gold early is not really eficient.

With creatures, perhaps not intelligent ones, I don't expect them to be "carrying" gold. If you kill a human (or humanoid) Brigand, then sure it'd have a coin purse. But no, I actually prefer harvesting resources from the giant spider and have to trade for coins later on.

This would depend on the gametype of course - if it's a simple platformer then sure, you can have every enemy drop coins or whatever currency you desire. But an RPG or something which has an element of realism then you shouldn't think that every "monster" even has the brains to appreciate shiny objects of value.

You are putting way too much thought into this.

If I get a rusty broadsword, a ruby ring and 30 gold coins out of a dead giant spider - I simply take these things and don't waste any further thoughts on it.

It's just the way it is - does it have to be "realistic"?
No, of course not - nothing about a game with giant spiders is realistic, so why should this aspect in particular be?

But - if you're so keen on giving thoughts to such things: why not simply expand your way of thinking and understand it this way:

the stuff you find after killing that giant spider was not literally coming from that spider, but was lying around in the immediate area around the spider.
And you were only able to retrieve it, after you defeated the spider.

Another, way more glaring question to me, would be:
why doesn't weigh gold anything in most games?

Only very few games apply weight to their currency (no matter the material).
Therefore, you can carry around amounts of gold, that no man would ever be able to carry.
Definitely not without getting massively hampered in walking/running speeds and/or combat.

Edit:
I forget to add...

there are basically two systems when it comes to how gold as a gratification in games gets handled:
1) the instant gratification - giant spider drop 30 gold coins, instantly available to spend
2) the delayed gratification - giant spider can get "plundered" for stuff that can be sold for 30 gold coins.
That way, you first have to find a dealer to sell the stuff to, to get your gold.

Of course, in many (most?) games that also means having less empty space available in your inventory for "more important things", until you sell the spider stuff.

And we all know, that can mean having to make the decision, to either drop the spider stuff to get some better piece of weapon/armor, etc - and therefore forfeit the 30 gold coins you could get by selling the stuff - or to keep the spider stuff for the prospect of getting 30 gold coins in the future, but for the price of having to forfeit that better weapon/armor.

So, which way a game goes for its gratification system, can vastly change the way you (have to) play a game.

An RPG need not have an element of realism in it. In fact, if the RPG is meant to be simple, it may not be such a good idea. (Imagine if Final Fantasy Mystic Quest had this requirement; it would feel very out of place in that game, especially since there really isn't much to buy in that game.)

While it makes no sense, it also seems to be a waste of time to force players to sell "junk" or vendor trash. If items exist with no other purpose than to sell for cash, just drop the cash.