well, obviously doing it yourself is just cost of materials: pads, rotors, fluids, fluid pump, some tools, jack...there's a DIY on pad replacement on this site.
http://www.m3post.com/forums/showthread.php?t=246116
There's also a DIY on fluid flush.
The floating calipers are much simplier than the fixed calipers, if you don't mind getting your hands dirty.
I can't find a DIY on the rotor part, likely it's just a simply unbolt/bolt affair.
At a shop, it's basically, a 2-3 hour job? So they'll quote you at their hourly rate X 2-3 hours and you have your answer, plus the cost of materials. If I were you I'd buy the parts from Tischer and bring it into a BWM indie shop. You can shop for the cheapest price on parts, and the indie shop is alot cheaper than dealer.
If you're a prospective M3 buyer, a lot of people were able to get a pad change for free within their free maintenance peroid.