Folks gave you the answer already, but given how different publishers clearly have different terms ad conditions for promotions with Valve, I further suggest you go here, choose a game, tadah. Obviously for new releases you will not have a long history of prices...

http://steamsales.rhekua.com/
Nice, funny :)

For a serious reply, the thing with Humble and Steam prices is that I know I might be throwing away money and losing access to my "licenses" in the future, but the price is right... GOG might be more expensive (maybe?) which to me makes sense as I'm getting something more durable...

Anyway, since GOG also jumped on the sales bandwagon, their normal prices do seem to have gone up. Sure that could also be misperception from the ratio of newer vs classic releases... Actually it probably came from the currency flexibility improvements related to regionalization.

Anyway, I do ramble. My point being that like the OP, it's the fact games are not on GOG that allures about Steam. And the achievements, cos who doesn't have a bit of OCD in them... oh so preciousss...

One ring to rule them all, heh? ;)

If I remember correctly, Croteam doesn't want to bring any more of their games here, citing the lack of a client, as well as a significantly lower userbase, compared to Steam. Maybe they will change their mind, given enough time, but I wouldn't be holding my breath of this happening anytime soon.
Serious Sam 3 is usually discounted by 90% during the big sales and by 80% on less big ones. Don't accept a lower discount. Duke Nukem Forever and Max Payne 3 have been discounted by 75% pretty frequently, and Alice: Madness Returns has been discounted by 75%. Note that in the case of Alice: Madness Returns, you'll need to activate the game through Origin.
OK. That's interesting.

Much the same as non-sales. 90% off something you don't own is still SweetFA.
But at least you'll have the warm glow knowing your details ill be used aggressively for marketing purposes.

The Steam sales are like GOG sales, but with more games. The word sale is meaningless these days. Every day there's a sale.

Yes, Max Payne 3 has additional DRM but it is not annoying based on my personal experience. Once the game has been activated on Rockstar Social Club, single player can also be played in offline mode.

I'm not interested in buying those games, renting's good enough. And paying 10% for renting something is better than paying 100%.

Ya. Scary stuff.

That's totally cool, and I respect everyone's right to do what they think is best. We all have different backgrounds, experiences, and aims, so live and let live I say. It sounds like you had a really bad experience with Steam and I imagine others have as well, and I can fully appreciate that and in many ways applaud it. Like ZFR, I basically treat Steam games that use CEG/DRM as a long-term rental platform, and I also have an interest in the platform from a business/developer perspective researching the current PC gaming market and taking a broad sample of the kinds of things that do well vs the kinds of things that flop.

Another tool for checking pricing history is steamprices.com .

I'm rarely hyped about anything but I actually am kinda hyped about GOG Galaxy.