I am old school and used to buying a full exhaust for the fox body Mustang. You could get a complete dual exhaust, including headers, for $600. Actually I am looking into a replacement exhaust for my '02 diesel, turbo back, and am getting prices under $300.
A 2.25" exhaust should work great. My '03 Cobra that was street tuned at 529rwhp only had a 2.5". That is about 265 per side. Also, it has been a few years since I checked, but the diesel truck land speed record holder only had a 4" single exhaust and is 4-5 times the size and makes 10 times the power. Keep in mind if you make the exhaust to large you gain a very small amount of hp but lose a lot of torque.