It's probably $16 at GMG still right now, but here's another place to get it for $16. It'll redeem on Steam and is Steamworks, so should work for Mac as well I think.
actually, the gmg sale ended a while ago, so this is the current cheapest . All the same, I won't be picking this up until it's at most 10$, and from somewhere that I have credit. Thanks anyway for the heads up, OP
You should add that this is for the PC in the title.
EDIT: Looks like my comment was transferred to a post made before the one I commented on. I original statement is longer valid and I look stupid. Damn you Cheapy!