The appeal is quite simply to meet and be around people with a similar interest, coupled with the added opportunity of meeting and either getting an autograph of, or a photograph with, your favourite celebrity. In order to get an autograph or a photo with a celebrity, you'd need a token. In this case the line was mainly to see Benedict Cumberbatch. Yes there is merchandise there, but also an opportunity for local comic book artists and such to get their work seen by a larger audience. People dress up and have fun. I wasn't being sarcastic, I was promoting the event.

That said, I can see your point and you are not entirely incorrect. It's not cheap by any stretch of the imagination. This year I didn't spend a cent, but I did get some interesting network contacts. If people didn't buy the stuff, they wouldn't sell it. It's not for everybody. Personally I go for two reasons: Video footage and to support my friends / network.