You should browse the "New and Preowned M3 Pricing, Ordering, and Tracking / Dealer Reviews" forum. Lot's of great information on pricing, leases, invoice, etc. in there.
Typically, anywhere from $500 - $1,500 over dealer invoice is a great deal and very common around these forums. Most dealers are aware of the going rate of these cars. This also depends on a few factors: build slot availability, the number of dealerships in your area, etc. Here in Southern California we have 25 dealers within 25 miles and you see $500, $800, $1000 over invoice without a sweat.
Some people will complain and moan for a few hundred bucks on deals, but the MOST important aspect of any deal that involves a large purchase such as a $75k car is how you are treated and taken car of POST SALE.
Buying from a dealer which is neither local nor reasonable for a $500 savings is not a wise move. Buy from a dealer where you have a relationship, or in your case are just establishing one, a dealer that will take your calls, provide loaners, pickup your car, provide you with stellar service and not nickel and dime you on everything for the period that you own the car.
I would have gladly paid $2,000 over invoice to my dealer for the stellar service I receive compared to SO MANY dealerships that just don't care about you or your car. Which is terrible and should certainly not be the way business is done.
'12 CLS550 Palladium Silver - loaded
'11 E92 M3 ZCP - SG/B-EXT/CF, M-DCT, loaded
'11 E93 328i, BSM over Saddle Brown, sport, loaded
'08 550i M-Sport, Carbon Black (Gone but not forgotten), '06 W211 E55 AMG (Gone, miss this animal),'01 E39 540i Sport