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. Baseballs authenticated by Steiner and MLB with Holograms sell at the highest price.
There is no way of knowing what something will be worth in 30 years. The value of a collectible is determined by supply and demand. If in 30 years there are more collectors in the market than the number of Derek Jeter signed baseballs available on the market the value will rise. If the demand lowers the value will lower or stay the same.
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