You're paying for the liscencing fee, the translation fee, royalties to the Japanese artists, and for the English-language design/layout artist in the process. The original artist needs to get paid, the original publisher needs to get paid, the company handling the liscence needs to get paid, the distributor/publisher needs to get paid, and the book store needs to get paid... that's a lot of people with their hands out for one little book... and that's part of why the price is so high.
The other is that the audience for manga is rather limited... and thus the price of a liscence/manufacturing run needs to be offset by a higher product price. Sort of like what happens when you try to split hotel rooms. 2 people in a $400 suite sucks... but put 8 in and it's $50 a night for everyone.
If you want cheap comics: learn Japanese.