Wow, I'm surprised Billy Dee was doing autographs at all, not only doing them that cheap and comparatively the same price as Mayhew.
but that's how this autograph/convention thing works. Buying autographs from dealers is kinda cheating. Half the thrill of a convention is getting the star to sign (whatever it is you want) in person, exchanging a few words and maybe having someone take a picture of both of you together during the signing. Makes the end result much more personally valuable and sentimental which can mean more than any monetary value.
Of course, there are those who are no longer with us (Nimoy, Doohan, Kelly, Montalban, Barrett-Roddenberry, Leonard, Whitney, etc) and i'm sure as time goes on the price only goes up. And there are individuals who do very few conventions or autograph sessions (Lien, Ryan, Blalock, Bakula). I would think these are the ones you would want to collect and try to find the best value for. You probably wouldn't want to pay too much for individuals like Takei, Wang, Dorn, Shimmerman, Phillips, etc who do conventions often or, frankly, aren't in high demand.