Except of course the cost of using Steam, which is far too high. GOG doesn't require installing godforsaken bloatware on your machine.

Especially on the internet, you'll find various places with differing prices.
A match in pure $ value match is hard to do as each service offers different features and services.

Emphasis mine. So what we have is two different products at two different prices. Color me surprised.

I would rather get it from GOG. Thanks, but I already have a boatload of crap that runs in the background no need to get Steam involved in the mess. Besides, you don't need to be online to play the GOG version.

I have Steam, but I still bough Arx from GOG, because GOG's service is so much simpler. Download exe, install, done.

...and that's where your sarcasm fails you.
Both Lure of the Temptress and Beneath A Steel Sky are available for free and still come with the same goodies that every other game here comes with (wallpaper, manuals, icons, avatars, etc.). So it's fair to say that the goodies are free with all games (otherwise the games would cost $5 as a "goodie tax").
Since DRM actually *costs* a company money to implement (as opposed to omitting it), not including it should make it cheaper, especially when the game didn't have DRM to begin with... so we'll call that issue moot as well. (By all accounts, the Steam version should cost more).
Now we're back to where we started: the original bits (Arx Fatalis) + nothing of consequential value. GOG is twice as expensive as Steam for this title. I'm not saying Steam is better, I'm just saying they have it cheaper, which some folk would appreciate knowing.

Sigh. Trying to analyze prices through looking at the minutiae of costs fails because effective pricing is not driven by what it costs to make something, but rather by what people are willing to pay for something. Perceived customer value is what is relevant to the features of a product, regardless of what those features cost (or don't cost) to implement. So once again we come down to two products, with different value as perceived by customers, and so it's not particularly surprising that they are sold at two different prices.

Two problems. First, you are assuming that the cost of production dictates value. It is a factor in price but it doesn't determine it. If I add an ugly spoiler to a car, it will increase the cost of production, which should increase the price, under your logic. In reality, I will likely have to lower the price to attract customers. The appeal is lessened because consumers do not want this feature. They might pay extra for an equivalent car that lacks this feature, if they hate it enough. Some may even get an inferior car for the same price, rather than put up with the offending spoiler.
Second, DRM doesn't create big recurring costs. After the DRM is added, it's on every copy. Once that money is made back, it has no bearing on the cost of production. I'm sure Valve has made the process as cost efficient and streamlined as possible. It could be entirely automatic, for all I know. This just leaves things like potential updates to the DRM software(diffused among all DRM products), and running Steam itself. A cost they would bear without DRM.
Nothing you value. The ability to play the game when and where I want, without another process running in the background is worth another $5 to me.

Correct me if I'm wrong, but it looks like you gave the game 1 out of 5 stars. I certainly hope it's not because of what you commented above. If you played the game and found it awful, fair enough.

It's 5€ on Steam, actually, if you're not from the US or UK. And yeah, goodies + no DRM = added value. I haven't bought the X-COM pack from Steam for 5$ (putting the euro conversion aside for a second), but I'll be sure to pick up the original X-COM: UFO Defense for 6$ from GOG as soon as it's available.

I don't quite see what the hate over Steam is...I think it is a great service and have used it to buy serveral games. I've had it installed on my system since Half Life 2 came out and have had no problem with it. The games all work fine with Steam running in the background, and you only have to launch steam to run the games you bought. You can still download and install the games onto any computer...also, like Gog, Steam usually has good sales too on wekeends (last weekend I picked up the X-Com pack for $4.99 and this weekend they have Rainbow Six Vegas 2 for $9.99.)
That's not to say that I like Steam better than Gog, I like Gog's service better than Steam...at the same time I think Steam is as very solid service and has come a long way since it was originally introduced (and I DO remember it having problems in the beginning). I think there is a place for both services.
I've also never had any problems modding games with Steam either...all the games in my ID Software Super Pack are just as moddable as the original cd's the I have/had (lost a lot of them for the older ones).

Steam, like anything, has good points and bad points. I don't hate it the way I used to. (Mostly because of the startup problems you mentioned. I couldn't get HL2 to work for days and Steam experienced numerous hiccups after that) It patches your games, which is nice. You can quickly and easily buy, download, and install games in one integrated package. The friends feature, now that it finally WORKS, is nice. The multiplayer features are great.
It has drawbacks though. I can't keep up with computer tech the way I used to. I just don't have the cash. So every extra process can be a burden on my aging rig. (Old enough that my graphics card is actually dying. I can count the number of components I've had long enough to plain old die on one hand.)
Many of the nice features of Steam are pointless for older games. How many more patches will Arx Fatalis get? Fallout? I can't play either of those multiplayer. So I have a program sitting in the background, doing nothing...
Except acting as gatekeeper. I don't get to play ball unless I do it through Steam. You can play offline but you still have to ask the gatekeeper first, and you still need to use Steam. I might be alone but I hate this sort of thing. Sort of like iTunes and iPods. I like iPods but I detest iTunes. Apple by gawd expects you to use it though. (Yes, I am aware that you can use other programs to manage it, I use Winamp, but firmware updates are only available through iTunes or via workarounds)
So what does Steam offer me, as far as older games go? Convenience, which I can get right here.

Steam is apparently still 1.18, but 1.19 mainly fixes graphical errors (notably the coronas showing through walls). 1.19 does not appear to be any more stable.
[Edit: Factually erroneous!]