For me, movies have gone up exactly $1 in the past 10 years. So, let's assume that the average movie price is $10, and has gone up $2. So suppose, for example, spiderman makes $400 million total in the next few months. Divide that by 10, you get 40 million viewers. Multiply it by 8 and you still have $320 million. It's not just the rising ticket costs... it's marketting and other factors.
Regarding harry potter, don't forget that kids get into movies cheaper than adults... so maybe more adults saw spiderman.
Regarding the actual quality of the movie, I thought it was equal to the first batman. In other words, very good. True, some of the special effects looked CGI-ish, and many did not involve / enthrall. But the story's execution was executed superbly. Just enough comic book feel to make it seem like a comic book story, but not so overly cheesy to make it seem stupid.
The best comic book adaptation, IMO, is HBO's animated adaptation of spawn. Not including japanese manga adaptations of course.