some more good (and cheap) games:
- eternal darkness - great story and some "fun" insanity effects.
- viewtiful joe - cool special effects
- pikmin (the second one should be coming out soon)
- super smash brothers melee >D
other single player games:
- metal gear solid
- zelda
party games:
- mario party, mario kart (not bad even with single player), mario golf
- super monkey ball (the story mode is sometimes too frustrating, though)
As for prince of persia (and also "beyond good and evil"), those two are quite underrated but nice games. However, my complain is that the games are short although they have nice story to it. i'd recommend renting them out instead, although for 20$ it's not really a bad deal.
check out the gamecube list at hot deals forums.. it's quite outdated, but the latest deals should be there on the last few pages...