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davidcyclebackFor an average grade card (Good to Ex), an autographed card will usually be worth more than the unsigned card-- that's whether the player is Mickey Mantle or Don Aase. For the autographed card, the value is dependant on both the value of card and the rarity/popularity of the signature.
You often have a different audience (with overlap) bidding on one or the other. An autograph collector may not be an autograph collector, but will bid on a 60T Mantle if it's autographed. A 1960 Topps collector may only be looking for an unsigned Mantle card. It's the card-only collectors who focus on the Mint 9 and Gem Mint 10 stuff. To the autograph collector, it may make no difference whether the card grades a 5 or 6 ... The two types of collectors will often use different critereon for judging the quality and value of the card.
For the card collector looking at a signed card, he should consider the value and rarity of the signature-- including how it relates to the value of the naked card. A Ted Williams signature can raise the value of a 2 cent reprint to $150. On the other hand, adding a $5 2005 Bob Feller signature to a Gem Mint 10 1938 Goudey RC might not be a good idea.
I do deal with autographs, including autographed card, and can tell you that an autographed 1960 Topps Mantle would be popular on the market. However, I don't follow or have interest in the prices for grade 8,9,10 cards, so can't compare the value to a plain PSA 8 card.