Have you considered our cat-back for the E90/E92 M3 (part #15545)? It should typically meet your price point (depending on your dealer of choice) and we think it looks pretty good. The sound will be a smooth, deep tone that is not much louder than stock at idle and cruise but comes alive when you put your foot into it.
http://www.magnaflow.com/02product/d...rtnumber=15545