I have noticed that shops specializing in M3 / BMW will charge around $300 because they can install springs very quickly. "Race" shops or those without a lot of M3 experience will trend towards $1000. It's a surprising range but, as with anything, once you learn the install "tricks" you can do the work much quicker / cheaper.
'13 M3 - Silverstone II - Sold
'04 M3 - Jet Black - BBS E88, KW Clubsports