There's been a market for signed guitars (or is there?), junk Squiers and Esteban's signed by Tim McGraw, Keith Urban, who knows, and some - P.T.Barnum said "there's one born every minute" - hmmmm, people have been trying to sell these things on eBay or CL for ages.
I suppose these days, Joe Bonamassa's signature must be worth a fortune ;-)
It was sad one day years ago when a friend of a friend of a friend heard that I [supposedly] wheeled and dealed in rare guitars and sold them on the net, and they had something they wanted me to look at, and they invited me to their nice vintage trailer home (I'm not here to judge) and they showed me this POS black Squier Affinity that couldn't even stay in tune with a signature from one of the guys in Blink 182 or something, and they thought it was going to be worth like $800, they "bought it as an investment"... I didn't want anything to do with it, but had to let them down gently...
I'd say if it's somebody really important's sig, like Leo Fender (unless he did sign every guitar then it's neat but not unique, and doesn't add to the value), any sig from Les Paul, Leo Fender, Willie Nelson, etc, etc would be of added value to the guitar, just how much, who knows.