There used to be an extra $2 surcharge on 3G downloads abroad. This changed a few months ago - downloading single, items (like books) are now free abroad
You will have to pay an extra $5 a week if you want to download periodicals/blogs etc if you want to use the 3G on those abroad, but if u don't pay the $5 then those periodicals/blogs will automatically download for free when connected to a wifi network.
So basically, downloading books abroad on 3G - free
Get the 3G model though - free wifi hotspots in Aus and NZ aren't as popular as in the States...