I'm no expert, but I'll try to explain. Firstly, consider the fact that I am talking only about U.S.-spec models. I have been a member of various WRX/STi and Evo message forums since both cars' introductions, checking them very frequently. From my understanding, the U.S.-spec STi can easily make 320-340 hp, but anything beyond that begins to require a lot of tuning and big mods beyond simple bolt-ons, including the front-mount intercooler installation, which can easily eat up $1000. (Keep in mind that the U.S.-spec STi motor has really only been around since 2004, whereas the U.S.-spec Evo motor is just an evolution of the 4G63 that has been used for what, 13-14 years? This means that tuners already have a pretty good understanding of the motor and how it will react to certain types of mods.) Simply put, I think the parts for the Evo are cheaper, and the mods necessary to get to certain hp levels above 350 are fewer. In addition, the Evo motor is known for being durable and handling assloads of power, whereas the U.S.-spec STi motor is susceptible to things like pinging and the ECU pulling timing when mods come into play. When I get home from work, I'll do a bit more research and post it here if you want.