You may have already noticed from my magic card collection, but I prefer games on a budget. Go into any game store worth its salt, and next to the $50 copies of Tannhauser and Kingdoms are usually a few $10-15 dollar games like Hey, That's My Fish! or Cold War: CIA vs. KGB.
Normally these games are a little smaller in scope, but that is actually a bonus because more people will probably be willing to play with you!
The love is dying down a little now and it's no longer on the front page, but a hot Geeklist "I'm a poor boy from a poor family" was chronicling these games for posterity.
From my own budget perspective…easily the most played game in my collection is Carcassonne. If you count all the games I played of Magic in high school, Magic easily comes out on top. But looking at the last year or so and it's medieval tile placement all the way down.
If you asked me what game I wanted to play, I would probably never say Carcassonne. But it's a game other people love, and I enjoy enough to play with them. A good time, every time.
I think the current going rate for the base box is about $20.