My zt was until last month still on the original exhaust and even now I've only just had to replace the back box. The back box made it to 129k and even in this state it still wasn't blowing as the outer shell was the only thing that was rotten! I know it was the original as the fitter had to cut it off as it was all one piece.
Personally I'd say your garage is trying to rip you off as any decent place should be able to cut and fit a new part.
Rik.