Consider a bunch of ~$10 retail games. I've bought a bunch from Newegg. Click around and sort by price!
On live arcade, my choice games are Bomberman, Aegis Wing (free!), Boom Boom Rocket, Castlevania SOTN, Catan, Chessmaster, Ikaruga, Megaman 9, MVC2, Metal Slug 3, Pacman CE, Puzzle Fighter HD, SSF2T HD remix (I got this one on PS3, tho), TMNT Arcade, Triggerheart Excelica, and Worms. I dunno how much each of these are individually, I just know that they're probably my best buys. Don't forget you can grab a demo of each one.