An '07 is going to be worth less than that the '08 he is looking at. So, if anything the '08 he is looking at is a better value than this '07 since they are the same price.

OP: according to KBB (not the standard but a decent starting point) somewhere around $40K is about right for an '08 3.0.

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2008 X5 4.8
Jet Black/Black Nevada/Dark Burl Walnut
Adaptive Drive
Premium Pack
Tech Pack
Climate Pack
Sport Pack
20" Wheel Option
Premium Sound
Sirius Satellite
Rear DVD Entertainment
Multi-Contour Seats
Comfort Access
3rd Row Seat
Heated Front Seats
Running Boards
3M VentureShield Paint Protection Film