It shouldn't cost more than 6 to 7 bucks to mail a DVD across Australia.
True cost is nearly half that, (assuming volume based whole sale rates with AU Post of course).
I think we payed about AU$3.50 for a DVD sized package inserted directly at one of the mail centers.
A lower volume rate would be around AU$4-5.
That said, it's not uncommon to run a low margin on your product, just enough to recover costs, then generate margin and profit through shipping charges.
For example B&H want US$50 to ship two compact flash cards to NZ.
I used to drop ship books, DVD's etc out of the US, so I know it costs only a fraction of that. They will be paying USPS US$2-3 and might incur a few dollars in picking/packing cost.