gizmo,
I can understand the interest in buying ONLY BMW rims but there are so many other options that will look similar but be significantly less in cost.
Plus, then you do not have to get 21" tires which will probably be run-flat.
Here are a few examples of the five (!!) sets of rims I have for mine:
19" BMW Original rims with summer run-flat - $65k! hehe
18" Corsa rims with Bridgestone Blizzak. New $1700
21" BMW M 7-series rims with P-Zero run-flat. Take-off for $1500
18" Bremmer Kraft with Yokohama Geolandar AT (they look awesome!). $350 used
I just sold the 21" for $1800. None of the rest are for sale. Just giving you options!
Paul