I've found that realoem does a good job, but the prices are sometimes, well, always realoff.com. Imagine my surprise when I tried to order a 2bbl aircleaner unit I thought was $45, when it is now actually over $900 at the dealer. No kidding.
I use realoem to look up the part numbers and study the diagrams, sometimes I take this list to my dealer, and recently I took a chance on Bell BMW's (NJ) online part ordering system with rather good success. http://www.bmw-auto-parts-dealer.com/
If nothing else, looking up the part number on real oem, and checking it with http://www.bmw-auto-parts-dealer.com/ should give you the current list price. Bell was great one some prices, like a tool kit roll for about $45 when my dealer wanted almost $80 If your order is big enough, the discount they provide will more than pay for the shipping, which is about 20% - no matter what you buy.
I've tried to use, and wasted many-a-man-hours with the mobiletradition parts ordering system....I think it was designed for people on another planet--it makes no sense to me either.
Ben