The value of the car at auction/private treaty will depend on its condition and model.
A V-Spec II will cost quite a deal.
An N1 will cost a whole lot more than 3M Yen.
You're also putting your trust in your reputable importer having reliable guys on the ground to do all the checks and balances and then tick the boxes.
Because there are set fees on freight, insurance, tax, compliance, registration etc, the more valuable the car is, the more worthwhile it is to import. And of course the corollary is; the less valuable the car is in Oz, the more worthwhile it is to buy it locally.
PM me if you need to ascertain costs.
Cheers