From what I remember, Adam and Juliet were $30 a piece for autos and about the same for photos. (Adam might have been a bit more.). I had a group photo with Juliet, so prices were a bit cheaper. Definitely go the group photo-op route if there's a few that you want a photo with.
I was surprised at DragonCon. Some actors, who I thought would charge a lot, were extremely reasonable. Michael Rosenbaum charged $20 for autos. (Dirt cheap, IMO.). His photo-op was cheap as well.
So long story short, you may get there and be surprised, but I'd expect the $30 range.