You must be buying them used, since new volumes usually go for 600-1000 Yen or more. Even used, the newer volumes are usually 500 Yen or so, although older ones can be as little as 100 Yen. There's lots of used bookshops though, so it isn't too hard to find them.
The most annoying thing about buying manga is that you can almost never read them in the shop (they are plastic wrapped) and the titles and cover illustrations don't really tell you much about the contents--even if you read Japanese. As a result, you can waste huge amounts of money on stuff that you really didn't want if you try to buy new manga.
Where are you in Japan? In Tokyo?