Over 300 whp on the wrx trans is right on the edge of its limitations depending on how agressive you drive it. Wont hold up for sure with hard launches, hard shifts, and driving like a ass in general. The cost of a turbo and supporting mods depends on if your buying new or used (big difference there). You may be able to get around 300whp on a vf series turbo which can be had for 300-500 used or close to a grand or better brand new. Supporting mods needed would be downpipe (100-350), catback (200-500), injectors (?), intercooler (300-800), tuning platform (not sure how much for a open source cable and laptop or 595 I think for a accessport. Most tuner shops will have a cable to flash open source with), and a tune (250-500). At the least a good boost gauge would be good to have as well. You also need to factor in maintenance cost if the car is due. You dont want to go modifying and pushing a car that has been neglected of regular maintenance. You will probably want your suspension and brakes up to par with the added power as well. All these prices are just off the top of my head estimates from low to high end of the spectrum. Someone els can chime in if I have forgot something or am misinforming you. Fast, cheap, reliable....pick only two.
06' wrx tr