There are many Jewish books for sale on Amazon as well as ebays half.com at reasonable prices. You may also find some decent prices for used Jewish books on abebooks.com or alibris.com. You can also try searching specific book titles in google shopping and see the websites that come up.
there is a great site just launched cheapjewishbooks.com it has just a few books so far – i hope they add more books, but the books they have are extreamly cheap- brand new and much cheaper than even amazon! good luck.