Would highly recommend
http://www.mrmcar.com
I personally haven't been in a few years, but he did some work on my M-Coupe. Well respected, specifically among E30 and M-Coupe communities).
I know he always focused on Ms, but according to the current website seems to work on a wide variety of BMWs.
Can't comment on current rates though. I would guess he beats a dealership, but maybe not the outright cheapest independent.