Posted By:
Rhett YeakleyBrian, the answer to your question is 100% dependent on which set we are talking about. If it is a widely collected set with a fairly well-established checklist then a uncatalogued card could be worth a small fortune. If we are speaking of a set (and this sounds like it is the case here) that is very thinly collected and a checklist is still a work in progress then there will be very little premium attached to a new card discovery.
An example of the latter would be something like a new George Burke stamp or something along those lines. The checklist is wholely incomplete and there is probably little interest by the general body of collectors when a new player is found.
Without revealing who the card depicts, what set is it? If we know this we can answer the question a little better.
-Rhett