I think it's a personal choice. I think an item signed around the time it was made certainly has more desirability thank the same item signed later. I don't think there is a set mark up per se.
My personal experience with 1934 goudeys is that I prefer period signed cards, but wouldn't pay more for say a Gehrig signed in 1934 vs 1936 vs 1939. I would pay more for a card signed in the 30s-50s in fountain pen vs one signed in the 1980s in sharpie. How much more? Depending on the card, maybe 30% more, but not much more.
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