Not to sure about costs yet, I assume that the more who show up- the better the price. This would be for an in-car tune, using your existing pcm. Although it is a good idea to have a back-up stock pcm in case you need to return your car to stock, so getting a 2nd pcm is a good idea. As far as GT tuning, I suppose there is always room for improvement, such as tranny shifting, tuning for different grades of fuel, tuning for better mileage. Hopefully someone with a tuned GT can chime in with better info for you or try contacting Will thru his "Overkill" site http://www.getoverkilled.ca/3800sc.html