I had a bimmer and it cost me nothing but fuel and tyres..had the BMW ultimate service package which literally took care of everything from oil, filter, plugs, break, etc etc..

buy new or get one which still has warranty + service package..after initial three years you can extend it up to a maximum of 6 years in total..previously it was not possible to extend service package for E60 M5 but now you can with the F10 M5