This game was already for 2,49€ some time ago on GamersGate, now it's in the same price on Get Games. Check it right here! That's a COOP must have btw.
Killing Floor is basically one of the best zombie shooters out there but remember to play it with a group of friends because you will get pissed a lot when you play on public games. But seriously to me this is the best game of that genre - Left 4 Dead sucks lol. Get Killing Floor here. It’s on GamersGate and is Steam redeembale naturally. Cheers!