Xbox has lots of cheap goodies.
Gun Valkyrie
KOTOR
Capcom Classics Vol 1 (Vol 2 is great too, but more money)
Blinx
Blinx 2
Buffy games (there are two)
Darkwatch
Jade Empire
Tork
Oddworld Stranger's Wrath
The two that hcman suggested are good as well.
Some might not be the best games eva but for the prices they typically command they are well worth picking up and playing imo.
Not exactly sure of current prices on these but I'd bet most are easily $10 or less. I actually just did $20 in trade (could have bought at that price) within the last month to pick up KOTOR and Cap Classics so that checks out I think.

Edit: The controls in Gun Valkyrie can be a little frustrating at first iirc but once you get used to them it's not too bad.