I've had most of the cheap preamps. Here's what I found:
Stay away from the Art Tube MP. Don't believe the hype, at least for a vocal pre. As a Bass DI it's really quite good, however.
The M-audio Audiobuddy will work just fine as a starter pre, and it's under $100.
For just over $100, the DMP3 gets raves. I have the Omni Studio which has similar, though not quite as good, pres and they are decent.
The Rane mic pre (I forget the model) has gotten raves in a lot of places - the MS1B maybe? It can be found for around $150, so probably another thing to check out (I never have).
Fab