A Touch of Jeff: Why don’t Comic Book movies sell Comic Books?
This is a good blog post. He cover's why comics books don't do as well as movies.
My answer, which agrees with part of his, is that Trade paper backs are the way to go! I love those, and will actually wait for a trade paper back rather than just buy a comic.
Cost is an issue too, a movie is far cheaper than many comic books and cheaper than a Trade paper back.