www.bookdepository.co.uk is the cheapest place to get books. Free delivery to your door (takes about 8-10 days to get to Australia).
They have launched a US version:
The prices are in UK pounds on both sites.
I’ve noticed that sometimes you can get even cheaper prices on the .com version. My guess is that this especially applies for books printed in the USA.
The range is excellent. And you get cool bookmarks that my family are buying books just to collect.