Can't tell you how much a shop would charge. You could do a GT catback though you'd just have to take it to a shop and have them make a mid pipe.
Honestly I would just buy one of the dual conversion kits from AM... probably going to save yourself a headache, get a better sound/performance and it'll bolt right up.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.