Here's my relatively unscientific profile of comic book price trends, in response to my prediction that mainstream comics will all be $3.99 by next summer.
This hastily-made graph on the left shows a sample of Amazing Spider-Man cover prices--from issue #55 to issue #555. I stuck with the issues ending in "55" because I figured that would avoid any double-sized issues and price problems. And I was too lazy to look up more than a handful of issues.
But you can clearly see the price trend, and by issue #655, scheduled for 2011 or so, the price should be at or above the four-dollar mark (if the graph continues in the same general direction). This graph doesn't account for the huge spike in oil prices which will probably drive the cost of EVERYTHING through the roof.