I know there's a big Steam sale post but this deserves its own post.
Castle Crashers is 90% off which caused the bundle it shares with BattleBlock Theater to be priced even lower than BBT itself! Grab it now while you can. The deal will end when they either change the price or in about 7 hours.
UPDATE
I browsed around on the Steam Discussion forums for BBT and it seems the developers are aware of the pricing and from their comments it doesn't seem like they'll do anything about it.
Damn it, I just bought Battleblock. Good find, OP
EDIT: Double dammit, BBT has been reduced to $2.49~