If you are willing to accept one that is a little rough around the edges, has high miles, or is otherwise a driver's car there are still deals out there. I consider the price I paid to be fair for the enjoyment that this car gives me. If you are more concerned about the value of the car than you are about the smile that owning/driving the car gives you, then it is time to sell in my opinion.
For me, I don't care if this car loses value from what I paid. Any amount of depreciation will be small compared to the depreciation I would have expected from any newer sports car that I could have purchased with the same amount of money.