let me break it down..
used JDM GC/GD front clip will run you between $3000-$5500 depending on year, model, condition and mileage. The onces that i have seen on the market are usually coming off the ver1-3 WRX in japan. their rated HP are ranging from 227-280hp. only down side is that its not smog legal in CA.
a used USDM wrx motor/tranny including ECU and other neccessary parts/hardwares will run you in between $4000-$5500. again, it all depends on mileage and condition. but smog legal if u live in CA..
then once you are done w/ the conversion, your high hp urges and boost junkie temptation will lurk you into spend another a couple grand just to be ahead of the game...(aftermarket ecu, fmic, larger turbos, injectors, and etc)
so far I have spent about over 10k including the EJ20 engine swap.
all in all..if you have the money to blow.. DO it!
