For what its worth, my son owns a M3, and when I asked him about cost, this was his reply.
"I just changed the oil last month, my 2nd oil change since I got the car. 1st time the dealer did it, also had the inspection 1 done while under maintenance warranty at XXX Dealer before it expired. No charge!! Inspection 2 is done at 60K and costs around 1500 from what I’ve heard. Oil change is about 60-75 bucks if you buy the oil and filter from the dealer, and is very easy to do (especially if you have a friend with a lift ;0) ) The M3 uses a specially formulated oil ava only from the dealer, Castrol 5w60 synthetic I believe.
The M3 is very easy to do because the oil filter is on top in front of the engine, and getting to the drain plug is the only challenge, which as I say is easy if I drive to XXX Location. I figure I need to get the oil hot any way, and XXX is very cool about letting me have lift time, if I give him notice, and come when convenient for him.
"I don’t feel the cost of maint is high compared to other marques, and if you buy new, it’s covered anyway, so why worry? My lubricated parts are covered for 6 years, or 100k due to the bearing recall done on this production series E46 M3."