The Jaguar XJS, the only way to own a V12 under 10K. (If you can keep up with the repairs.)

The Ford Crown Victoria ex. police cars are pretty quick. And can be picked up real cheap.

The Ferrari 308's are comeing down in prive nicely to about 20-30k

The Jaguar XJR's are starting to dip below 20k. I have seen them go for as little as 15k.

The best way to go fast for cheap is american iron though, the 5.0 Mustangs, Camaro's, can be picked up for a song, and can be pushed into the 11's very cheaply.