Japanese comic books are much different from their Western counterparts. Manga is and has been much more a part of mainstream culture in Japan; it’s consumed by children and adults, men and women, and cover a broad range of genres and subjects. Comic books, meanwhile, had a rough time in the 1950s, and even now still carry something of the “nerd” tag (though that stigma’s much less than it used to be). Nowadays, you walk into any major bookstore in the US and you can find a massive manga section — bigger than the comic section by far. (Japanese bookstores often have an entire floor devoted to manga, making even a US manga section look puny by comparison.)
A volume of manga in the US runs somewhere in the neighborhood of ten dollars. That’s very likely due to licensing and translation. Meanwhile, here in Japan, you can buy a volume for about half that, or one of the phone-book-sized monthly compilations that have one episode of maybe a dozen different titles for under fifteen dollars, and those often come with a miniature figure, hand towel, poster, or something similar. The latter books are printed on cheap paper, and cover both famous and untested titles. Ultimately, manga series are compiled into tankoubon, which are better quality overall and have multiple chapters in the same book. This means individual volumes lose their value quickly, which is why the used market for manga here thrives so well. It also renders those giant phone books largely worthless. It’s not uncommon to see them left behind in laundromats, for example, or simply thrown out in bulk.
Popular manga will ultimately be licensed for anime adaptations, of course, and many also find third incarnations as live-action movies. In some cases, the anime series will catch up to and pass the source material by (e.g. Naruto), or deviate heavily from the original (e.g. Fullmetal Alchemist, as opposed to FMA: Brotherhood, which was more faithful).
Personally, I’ve never really been interested in manga. Not sure why, but I was never a comic book fan, either.