I think it would depend on how many of the stars that were there. And if "characters" like Woody and Mickey, etc. were there for the younger kids.
But seriously, I would pay premium prices for the stars of shows and the JB. People pretty much paid like $75 for tickets to JB concerts, unless you had already paid for the original tickets and then you bought the premium sound check front floor tickets for $145 because you did not get floor after you paid for the fan club at $50 some odd dollars. Believe me, what you charge for fan fest would not be enough to ensure that you would get a ticket if the JoBros are a part of it.
But I guess I would say that I would be willing to pay between $75 and $100 per person ( in my group, not per Disney star) if JB, Miley, Sprouse twins, Selena, David Henrie, Jason Earles, Cody Linley, Brian Stepanek, Demi, Alyson Stoner, Jennifer Stone, Chelsea Staub, Brenda Song, Mitchel Musso, etc. etc, were there to sign. More, if they actually took some time to talk to my kids for a couple of minutes. But my only question is, "For the price, how many stars are my kids going to get to talk to?" Because that is the bottom line. They are commodities for Disney and I am a consumer. What am I going to get for my $ ?