What is a good price for a 2000 BMW M3?

That depends on whether it's a coupe or convertible, whether it's an SMG or manual transmission, mileage, whether it's a slicktop or has sunroof, whether it's had subframe reinforcement or rod bearings done, and whether the cooling system and oil-related gaskets have been replaced. BringATrailer is a good place to see auction results for well-maintained M3's.