Not sure about prices these days, but I believe these are cheap:
Winner is you:
GOMOLA SPEED (I love it, one of the most underappreciated games)
If you are like me (these games are not universally appreciated):
CYBERCORE (Sounds like you enjoy shoot-em-ups, this is a decent one--not spectacular--but I love the music and, and, I love the bugs. Sure, it could be a better game, but it's cheap!)
SPACE INVADERS (I like the "updated" version on this... of course, it you are sick of/not interested in Space Invaders, steer clear)
Beware of what the masses say:
RASTAN SAGA II (It truly is a crappy game, but, if you are like me, it might burrow its way into your heart. Like a parasite, I suppose. Give it a chance, there is some good masochism in this game.)
ROCK-ON (Another game--horizontal R-type-esque shooter--that is condemned for being a steaming pile of garbage. However, I found it somewhat addictive...I kept wanting to "get a bit farther, next time" and playing it and playing it.)