Yep, they are often pretty cheap! Some series can run a little bit higher than that, but not much! For physical books, you do have to watch out for shipping though. It’s often best to bundle purchases together as much as possible to save on shipping costs (either by buying full series together or buying multiple series at the same time, etc.)
For even cheaper manga, check out the Bookwalker Freebies Thread, for digital manga! That is updated once a week with a master list of all (currently) free manga on Bookwalker, and more often than not, if it’s free on Bookwalker, it is also free on AmazonJP! (I have amassed so many books this way… 
)
Edit: Found that guide to purchasing from AmazonJP, so just as well post it!