The Most Valuable Football Card Ever Sold Was One Of The 1894 Mayo Tobacco Company And Is Belived To Be John Dunlop Of Harvard. There were 35 players in the set from Harvard Yale and Princeton. There are only ten of these cards known in existence and one was sold on eBay for $10,101.51
If this is true, that would place the Dunlop only around #7. This list at the URL address below are realized prices and several of the cards blow this Dunlop out of the water.
See Related links below for a website that list the top ten.
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If I had to guess I would say it is a Walter Payton Rookie card that is autographed, and a jersey card.