When I was buying my 07, I used www.edmunds.com as a starting point. The TMV price shows what others in your area paid for the car. Not sure if there is a lot of data for 08 models yet.

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[B]2014 Cayenne Turbo S[B]
2007 X5 4.8 - Titanium Silver , Black Leather, Premium, Prem Sound, Sport, Rear Climate, Heated Seats, Running Boards, Lux H8, GP Thunder Fogs
2003 BMW Z4 3.0si (retired)