Though its not the case for some of the bigger guests(they are definitly paying Billy Dee!
), some of the guests may come to the Con and work only for their autograph fees, even paying there own way to get there.
At the Central Canada Comic Con guests will charge $20-$25 dollars per autograph, sign multiple items and usually pose for pictures for the price.
But each Con is different. I know at Calgary Expo pictures with the Guests of Honor were an additional charge, though most the other guests allowed photos with an autograph purchase.
Everything will depend upon the individual guests and the arrangments ironed out with the promoter before the Con.
