Solid advice from ard, IMO.
Another way: If you are able to get the MSRP for the specific vehicle (window sticker, BMW build-your-own etc) and I am assuming that you're looking for a diesel:
Take off $10,000 (aggressive haggling, with all normal incentives) off the MSRP. Then multiply the mileage with the $/mile ard gave and deduct that from the above and voila, you've got a price which should be close.
Example: a 2011 X5 diesel with 10,000 miles. MSRP was $60,000, it was probably sold for about $50,000 - $52,000. Say $50,000. 10,000 miles x $0.75/mile = $7,500. Used price $42,500.