Varying results really, some built full on time attack, some built drag cars, some just built road/track cars. One guy was building a 20b (triple rotor) rx7 and had the engine rebuild 3 or 4 times as he keep blowing it up with a bad fuel set up, your probably looking at minimum 7.5k each build, he ended up with 550hp from memory as he got to scared to run anything but pussy boost. Another guy did a Veislide, 20k just for a paint job and some fiberglass panels.
A mate has a body shop/garage, he was building a Supra for a guy, the "first" delivery of parts came in a 7.5 tonne lorry just for his car and was circa £50k, bride seats x 4 as he wanted the back seats to match, wheels, turbo, exhaust, hks fcon, abflug bodykit, bbk, the list goes on. Its been there 4 years, haha, every time he gets some more money, he turns up with more parts and pay off some of the work done
I have done with my modifying to that extend, i enjoyed the journey but when i got my first 350 it always surprised me how much i enjoyed the very basics of motoring, having a car in my garage that i could get in and just drive