I've never really gone to a convention of any size, so until recently (I think from reading people talking about it on here), I thought that celebrities who attended conventions made their money by being paid to appear and that their autographs were free or for a nominal amount. I had no idea until recently that they were charging these kind of prices for autographs. Personally, I wouldn't pay that kind of money for someone's autograph, but I wouldn't blame anyone else for doing so, if they really wanted the person's autograph.
I remember going to a smaller show, here in the St. Louis area a few years ago, and the guy who created Ghost Rider was there, signing autographs. He might have been charging a little money for autographs, but it wasn't very much. I felt bad for the guy though. It looked like most of the time he was there, he was just sitting by himself, waiting for someone to come up and talk to him. That's pretty much been my experience with people signing autographs at shows though.
I do agree though with those of you who said that it's pretty douchey of celebrities to refuse to sign autographs for kids in a public venue. I'm sure there are some situations were doing so maybe isn't practical, but as a general rule, it seems douchey.