Depends on which ones you get....
Getting all the books on the 'distance learning course' from Advanced Flight Theory will set you back over $2300AUD- but this is all of Nathan's notes, the best in the business!
Sounds expensive as you can just go and buy individual books from other authors (which are still pretty good), but with Nathan's you get all the practice exams and support too.
I spent my money on the Advanced Flight Theory books!!
Search for the pilot supplies shops in Aus and check out prices. One place to start is Skylines.