it would help if I knew what games you like and which ones you all ready have.
I would naturaly nominate my favorite game, Ninja Gaiden 2 because it got a price drop and is now just about $40. But even though it is pretty long, I'd actually say to get a game with a lot of replay (like a sports game, rhythm game, or a good online game), if you want a good investment, asuming that your small amount of cash is a sign that you dont get money too often. I would also agree that SMT games are good, but some of them are as expencive as a newer game, not to mention hard to find.