This is one of those things that cuts in favor of the reader, so don't think that I'm trying to complain here. I just don't get it, that's all. Anyway, to the meat of it.
In the U.S. marketplace, a volume of manga generally runs between $8 and $11 (some on the higher end come out around $13-$14), depending on publisher, size, and so on. As with any product, brought overseas and translated, the cost compared to the cost in the country of origin goes up for a variety of reasons (licensing, cost of translation, etc). In this case, that roughly doubles the price (400-800Y, ~$3.50-$7 as a rough estimate).
So, here's the thing that gets me: Translations of light novels (essentially, novels with a couple of pages of illustrations) are only running around $8 over here, while they're at 600-650Y (~$6) in Japan. All I can think is, companies are getting relative steals on licensing light novels, because I can state unequivocally that the amount of time needed to translate 300+ pages of full text is significantly higher than the amount needed to translate 190+ pages of occasional text, particularly when the art involved often lends clues to the translation effort.