It's always difficult to put a value on your own car, since you only see it with your blinkered eyes. Hence I recently asked the Forum for a census on What's it Worth - the general opinion was £9k. Hence the car was sold for very near that - no complaints by me, just to give yoy an idea the car in question was a "MINT 2009 Z4 3.0 si" Manual in Black with Black leather that had only done 21k miles.
A car is only worth what someone is prepared to pay for it, although the E85 is heading towards Classic Status.