You would have to put it up for sale. Then the number will show if you overpaid. Maybe someone wants it more than you and will pay a higher price. Or maybe you just had to have it and overpaid. That's the reality of it....
I just bought it and want to keep it for now but was just curious on the price. The car wasn't actually for sale when I bought it but i made an offer on it and came to some agreement.
