ENGLISH BOOKS ARE (NOT) EXPENSIVE.
We can assure you this is a WRONG notion in Vietnam. English books do have higher price range, only because they are printed on high quality papers and imported all the way from the US/UK.
A 400.000VND book is equivalent to a $16 book, which is considered as a good option in other countries, but there are still a lot of economical options. Check these books out if you just started reading English books, or you want to read something good with a budget in mind.