Help needed! What game should I buy for under €2.80?

damn, you just made ME buy Goat Sim Payday DLC

http://store.steampowered.com/app/222640/

Hard to tell, what kind of games you would enjoy as you mostly play CS. So I would assume you look for something nice to play solo that isn't like counterstrike?!

I wanted to recommend The Witcher, Mass Effect or Hotline Miami, but you already own them all.

So here are now just some recommendations of critically acclaimed games, that might catch your eye and that are around ~2$ cheap:

Bully: http://store.steampowered.com/app/12200/
something from the Double Fine pack: http://store.steampowered.com/sub/86537/
Brothers: http://store.steampowered.com/app/225080/
Braid: http://store.steampowered.com/app/26800/
Hitma Blood Money: http://store.steampowered.com/app/6860/

Flamingo Simulator!

Dang, thanks for that list of bundles.
Of those, I'd say Punch Club is a great game.

I assume you already have Plants vs. Zombies.