38-40k seems to be the right price point for an 08 car under 40,000 miles at a dealer. I paid in that range for mine and a quick Autotrader search across the entire country proves that. Thats for a non-CPO car so add 3k+ for CPO.
Looking at the Manheim auction transactions over the past few days dealers are still paying upwards of $36,000-$38,000 for the M3s in that mileage range with a high quality grade. They are not going to sell to anyone without some profit.
Definitely looks like you can get to the mid-low 30s if you're willing to break the 55-60k mileage mark.