Got a Magnaflow 14815, dual tip stainless steel, universal muffler, then had a local muffler shop do the installation and remove the resonator. It cost me $175 for the labor and my Z3 sounds fantastic. I also asked the installer to cut and flange my stock pipe so I can re-install the stock muffler and resonator myself, if I ever sell the car.
$200 for the muffler (Pepboys)
Total $375 ... not a bad deal