Impossible to answer.
We always say that they are worth whatever someone will pay for it...no more, no less.
The only way to get an accurate answer is for him to put it on e8ay and see how the bids go...but then you might not win it.
The real question is ...how much is it worth to you?
One of us will tell you it is worth $50.00 and one of us might tell you $500.00. It depends on how rare, condition, and how many guys want it. You know..the old supply and demand thing.
I know this is not much help...but it is the best I can do.