Value of car purchase? Please, it's complicated!

I feel this is a good juncture for your son to move into adulthood. Since it is his car he should decide on the price he wants to sell it to you. If it's too much, offer ONE counteroffer "the most your Mother and I can pay is $X.". If he doesn't agree to that then he can go ahead and try to sell it to the public. There comes a point as parents that you have to let the children handle their own affairs. If he doesn't do what you feel is right, don't hold it against him. He still has lots to learn about life and it's only a car.