See here:
http://realoem.com/bmw/showparts.do?...66&hg=51&fg=15
You want both sides of 15 in either silver (x-line) or black (sport line) and if you want the middle piece, it's 6. Looks like the silver ones should run you about $17/side and the black should be about $15/side. The center piece is about $50.
Edit - it also looks like you'd need to replace #13 on each side, which are about $22/side, and if you want the center trim piece, you'll need to replace #4 with #5, which is about $41. So all in, you'd be out around $75 for the side trim and $100 for the center.