Valkyria Chronicles is awsome (still can't past it though)
The Darkness is a pretty good, cheap game.
Dead Space is fantastic if you like Resident Evil type games.
The Chronicles of Riddick: Assault on Dark Athena is another awsome game, plus it has an extra X-Box game on there.
Yup, those are some games I loved that also happen to be $20 or below. Valkyria Chronicles would have the most play time though with like 50 hours.
Bet with Conegamer and AussieGecko that the PS3 will have more exclusives in 2011 than the Wii or 360... or something.