i had an 88 535i and now have a 1991 m5. i would seriously estimate the running costs including services/parts to be 50 percent (or more) higher. unfortunately the gearbox on mine went "SPLAT" earlier this week at a massive cost of £2000 sterling to rectify. always remember that any other model items are stock items and secondhand parts can be obtained usually. but M5 items arent. try finding any second hand items!! its very difficult. but so much for the downer!! the M5 is an awesome motor car and i would not be without it!!!!