Great piece.
Assuming everything is authentic (and I have no reason to believe it is not, but I'm also not an expert) I would think the place to start thinking about a value is looking at signed high school yearbooks for Koufax.
Sold 2015 at Heritage for a little over $500
https://sports.ha.com/itm/baseball/1...a/7135-83343.s
Sold 2011 Heritage (not signed until decades later) for $1135
https://sports.ha.com/itm/baseball/1...a/7041-80976.s
Didn't notice any others in a quick google search but may be a direction to think about this.