There are two remaining factors in collecting signed documents, and they really apply to any type of paper collectible: rarity and condition.
Collectors must remember that paper is fragile and so often does not survive. The very term for paper collectibles, “ephemera”, literally means something that is fleeting and does not last. That’s why when a rare signed document comes to market in great condition, the sky is the limit for serious collectors and records are continually broken.