Value is based on average prices of recently closed auctions. Prices may vary based on condition, and the type of authenticity that accompanies the signature. Signatures that have not been properly authenticated could sell at half the market value or less. Add for inscriptions
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Eighteen dollars.
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I find this answer to be dubious. No source is cited and its really impossible to assign a value without knowing about the condition of the ball and the signature and its authenticity. See the related question for more.