I think your going to have a hard time moving this dealer on price.
He is, obviously, infatuated with the mileage on the car and believes that's going to earn him a big return. He is probably going to wait for the customer who is also infatuated with the mileage number and will pay him top dollar for it. If he's had the car on the lot a long time, you might have a better chance.
The thing to keep in mind in situations where mileage is driving the price is that average mileage on a car is about 12000 miles per year. For an '08 that means the car could have 60,000 miles on it and it would be considered to have average use on it. That's not a bad thing, and you know the car has a lot more life in it. So, if you look at '08 M3s with mileage between, say, 40k to 60k, many start under $40k, and with a little effort, could probably be had for less (cars.com, autotrader, craigslist).
In your negotiations with the dealer you might want to acknowledge the low mileage on the car but mention that there are many other cars out there that have acceptable mileage for you, some with more options, and can be had for less than his car.
As far as how much you should bid on this car, that should become clearer to you after you've done a little analysis of the market. You might, for example, want to determine the "average" price for the "average mileage" cars, i.e., cars with between 40-60k miles on them. Bid below that figure.
That's how I'd approach the money angle.