If it's high end cards, you could track it over the years from the auction house sales. Say a particular card is in auction house sells for some price. The exact same card gets reconsigned to a different auction house a year or two later. Sells for a different price & so on. You could see the value that way. For the lower end cards, there's too many variables as has been pointed out earlier. For example, not too long ago, a card of a Jewish player sold for a rediculously high price only because two Jewish players collectors wanted it that night. If one of those collectors didn't see it on eBay, it would have sold for much, much less.
Alan
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