This instructable explains how to build your own 100 HP (peak) motor controller for use in an electric car or motorcycle conversion. It can take any voltage up to 144v, and the peak current is 500 amps. The cost of the components is a few hundred dollars, which means you can save over $1000 by putting one together yourself. At 144v, you can expect a top speed of around 75 MPH in a car.
Check out http://ecomodder.com/forum/open-revolt-open-source-dc-motor-controller.html
if you want to read about the whole story!
Experience in soldering is important. If you want to really keep costs down, a mill is helpful, but that work can be outsourced to a local metal shop.