I'm not sure how much it would cost. I would just say check with a couple muffler shops get prices. One of my driver's side cats is messed up too. Bottomed out. OUCH! It kinda leaks a little bit of exhaust some times, too.
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FOR SALE: 1996 Mustang GTS: 5-speed, K&N, removed air silencer, 3.73s, 49,000 miles. $11,000.
http://www.mustangworks.com/cgi-bin/moi-display.cgi?388