He stopped autographing things in 1974 when he realized such items were fetching good money. He was humble and didn’t want to ‘commercialize’ the accomplishment. As such, I would imagine his signature is quite valuable. There’s film/video of him politely refusing to autograph fan’s items.
Interesting. Former Yankee Don Mattingly stopped getting (paid) invitations to baseball card shows when the organizers found out that he’d autograph cards and other memorabilia for anyone outside the event who happened to ask him for it. Apparently it never occurred to him that his autograph was “valuable,” and that the whole purpose of setting up a baseball card show was to allow the organizers and attendees to capitalize on it. LOL.