Panzer Dragoon Orta for 6 bucks. Best thing Gamestop has ever done for me.

A close second was getting a Dreamcast for like $25, spending an additional $10 or $15 on controllers and a couple VMUs, and then being able to find every game for the system online.

One of my friends found FFIII/VI for the SNES in a pawn shop for about two bucks. That was pretty awesome.