Well to give you an idea on how prices vary.
The cost for a 200 gal
bucket of Instant Ocean salt at my LFS
that is just down the street is $70 8O where as if you drive about 45min you can get the exact same stuff for $49.99. It all has to do with competetion. My local LFS
has no local competition for saltwater so they can charge what ever they want to charge where as the LFS
that has it for $49.99 has a few other petshops that deal in saltwater in the same city.
Online I got the same bucket for $36.00 but had to pay shipping and by the time shipping was added it came to $53.80. The reason is the bucket of salt is very heavy at 60lbs per bucket so shipping is expensive.
Given its expense to ship I would think salt might be overly expensive in austrialia unless you purchase an austrialian brand or say Instant Ocean has an austrialian plant they produce it in for you guys down under.