Mafia 2 is on Steam and Amazon for $7.50 currently.
It’s also $7.50 on Direct2Drive. However if you use coupon code hangover you’ll save an additional 20% bringing the game down to $6. The Direct2Drive version does activate on Steam.
Update: If you purchase a $10/$20 discount code (you can purchase a $20 discount code for $10) on weshop you could pick up Magicka and another cheap game bringing your cost down even more. I have no prior experience with weshop however. I heard they’re pretty good though.
Link for the weshop promo. http://shop.weshop.com/products/direct2drive
The weshop code can’t be stacked with any other deals.