Don't know what door or side you are looking for, but here are 4 part numbers and estimated prices:

MOULDING DOOR FRONT LEFT 51-13-8-402-649 ($30.41)
MOULDING DOOR FRONT RIGHT 51-13-8-402-650 ($30.41)
MOULDING DOOR REAR LEFT 51-13-8-402-653 ($16.52)
MOULDING DOOR REAR RIGHT 51-13-8-402-654 ($16.52)

Use a heat gun or hair dryer and heat up the adhesive on the old trim to remove it. I also heard that you could use dental floss to cut the adhesive. I tried that one time and make sure you wear protective glove (ask me how I know). When you remove the trim, do it slowly because it is very brittle and will break.

And here's where I got the parts and prices from:
http://www.realoem.com/bmw/showparts...09&hg=51&fg=20

Good luck!