How do you figure what the additional value should be for a one-owner (50 years) car?
I apologize for the vague answer, but it is situational. If we are talking about a relatively uncommon to rare car where no irreversible upgrades were performed and original documentation was kept, then yes, it is reasonable to expect an increase to market value. Any bump in value is typically correlated with one owner vehicles that are also well documented. Having original ownership but lacking documents might not realize the small bump that having both would. What this does though is it eliminates the game of “telephone” that verifying a car can turn into. The less people involved in the ownership chain, the more likely the story is correct.
Typically we see cars with impeccable documentation and a known chain of ownership bring market values on the upper end of Hagerty Price Guide recommendations, especially when associated with a quality restoration or a high level of preservation.
On the other hand, a relatively common car might see a negligible change in value because of ownership due to the quantity available, but what it might do is increase the desirability of that one being the car to have compared to other offerings.
Thank you so much for taking the time to respond to my email.
I have lost many hours of sleep trying to determine a value.