The value attributed to a pedigreed card can range from 0% to many multiples of the cards regular value.
I know people who have no use for them and others (like myself) that collect pedigreed cards.
In this case of the Mayo owned by Casey Stengel, I paid a premium (maybe double the card's value at the time) just to have it.
While I have picked up some Lionel Carter & Frank Nagy's at or below book.
But if a different issue Carter pops up that I don't have, I would indeed pay a premium to obtain it.
