AFAIK, the console versions were converted to used at GameStop a while back, so you might be able to find a mint condition one around you for cheap if you have a preowned coupon of some kind. Otherwise, brand new copies are usually around $16 at Amazon.
For PC, wait for a Steam sale. It's been $10 (or $13 with all the alternate costumes) a few times there. (It's also been the same base price at Amazon/GamersGate/GMG in the past.)