It's not cheap. The M5 is a premium car after all. Despite prices falling quite dramatically, maintenance costs have not.
You can save yourself a lot of money by looking around to find the parts at a discount and doing the work yourself. No matter how complicated the car, some things are still pretty "normal." Such as changing the oil or changing the brakes.
The bigger worry with the M5 is electrical gremlins or transmission and engine failure. If I were you I'd pick up an extended warranty.
Definitely i would have to purchase an extended warranty from BMW but i dont know if they would give me since the mileage is at 58k now. I found an aftermarket extended warranty, Its bumper to bumper warranty, 4 years or 100,000 miles. $5,200. What do you think about that?