My dealer was worthless - wanted $650 for the resonator and tailpipe, including an estimated 3 hours labor at $85/hr. The major muffler places were a little better - $450 using Mercedes parts. I ended up at my local tire and suspension place getting Italian parts with a 2 year warranty for $320, and they installed it in 30 minutes for no extra charge.

My dealer truly did not know how much labor would be involved - I think almost nobody goes there for that type of repair. The Italian parts (I forget the brand name) were made specifically for the car, fit perfectly, and have performed well.