http://www.jbhifi.com.au/games/pc/
(please remove link if this is inappropriate)
Vanilla WoW at AU$19 is cheapest by far I've yet seen.
The BC WoW at AU$34 is a little more expensive than ordering online

If the battle chest is AU$39 each then that is again by far the cheapest I've seen.
hrmm... "Offer ends Monday 28th Jul 2008." at the bottom of that page...

I might check it out tonight on my way home from work anyhow