I'm fairly sure it comes from the costs of US Comic Trade paperbacks (several chapters of printed comics bound into one book), which are usually in the $10+ range, especially for nicer ones. (The Sin City trade paperbacks are all ~$17 from Dark Horse, fairly nice paper and printing. Of course, there are also the Marvel Trade Paperbacks, which have very nice paper (glossy & in color) and printing (though less nice bindings), and cost around $8). Most of the manga translation and distribution is done by these same comic publishers, so they expect to see the same kind of returns. Since they're distributing less volume than the Japanese publishers, to cover the costs of transportation and printing, they charge more.