short answer:
they aren't that expensive, how much silicon do you think you should need to control a half megawatt motor?
right now you can get the basic series wound motor controller for about one cent per watt.
if you can find the igbt's on ebay or surplus sites for 1/4th the standard price you can build your own and save some serious money.
If you want to take the risk, try paralleling 100, IRFP460 mosfets, as you can get these for about $1.39 a piece, and this would get you a series wound dc motor driver rated to about 350v @ 300 amp for about 400$ in parts, but it won't have regenerative braking.
Alternatively, the same silicon could build you a 25Kw 200-260vac 3 phase induction motor drive, and that would be a lot easier than making a 200Kw buck regulator.
but then you better be able to play around with 50mhz processors just to handle the PID...
an alternative to ^that is find an older totaled honda civic or such hybrid car and buy it off the insurance company. i don't know how much money the older inverters are still getting, but the geico agent i talked to the other day said he's never heard of them ever failing in the vehicle, so i should be able to get it for pretty cheap. (dunno what that means though)