I buy many of my games used.
Redflagdeals.com has a great trading community. I'd recommend looking there first. Kijiji.ca is also a good alternative, depending on where you live.
As for recommendations, I'd say Pixeljunk Monsters. Unparalled charm and a unique take on the Tower Defense genre. Plus it's only $10.
Disc-based? I know you're a SRPG fan, have you tried your hand at the Disgaea series? If so, Disgaea 3 was great and, I'm sure, can be found relatively cheap.