I'm not an expert on this topic, and I only have one autograph, Drew Pearson's obtained at a local Wal-Mart over 20 years ago. But for any acquired from retail packs I'd say just check out what's currently being sold on Ebay, Sportlots, etc. If I paid for the autograph, say at a card show, then I'd add that to the price. Logically any authenticated signature adds value to a card, and any number of factors can determine what that value is as you already know.
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“Aslan didn't tell Pole what would happen. He only told her what to do. That fellow will be the death of us once he's up, I shouldn't wonder. But that doesn't let us off following the signs.” - Puddleglum in The Silver Chair by C. S. Lewis