The value of multiple signed item cannot be priced by the value of each signature added up. Multi signed items are valued as a group of signatures, and how desired that group is as a whole. This does not necessarily mean that two signatures will sell for less than one, but in most cases it does, or at least not the full value of the two signatures added up.
Value is based on past auctions and sales. Value will vary based on the grade of the signatures, display, (eye appeal) and the type of authenticity that accompanies the autographs. Signatures that are not properly authenticated could sell at half the market value or less.
I did a quick search for "Multi signed home plate auction" and found a couple with a low number of signatures selling in the $500. range with Mays as the key signature, one with 57 HOF signatures that sold for $2,880. (following link)>https://goldinauctions.com/Baseball_Hall_of_Famers_Multi_Signed_Home_Plate_Wi-LOT29710.aspx
and another with about the same number of signatures that sold for $2,390. (following Link)>https://sports.ha.com/itm/baseball/1980-s-hall-of-famers-andamp-stars-multi-signed-home-plate-exceptional-compilation-of-sixty-9-10-and-better-blue-sharpie-signat/a/707-19227.s?ic4=ListView-Thumbnail-071515
Your plate with only 18 signatures can still compete with the high number signature plates. You have some great "Group" of key signatures, and as I stated above a Higher number of signatures doesn't always translate to a higher value. The Key signatures and overall display will play a big part.
The couple of auctions I posted here are both properly authenticated. If you decide to sell this would have to be done to get top dollar. If you look for more auctions to help find a value, take note of the key signatures, Grade, display, and authenticity. Do not use items that did not sell yet to estimate a value. -Steven KeyMan
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