If the figures quoted are to be believed.....
The unit price for a SuperHornet is around USD $56 for a USN delivery sans weapons. We're buying 24 so at that price and allowing for the exchange rate that would be 24 x 72 = $1.728b AUD.
What about the remaining $4.272 BILLION?
It doesn't state whether this is adjusted back to '2007' dollars or whether this includes expenditure inflated 'future dollars' up to 2017. There could be all sorts of other procurements in the project including new buildings or runway upgrades or what other 'goodies' they can get included in the scope of this additional funding. There's probably also a 'contingency' built into the program funding for unforseen costs and delays.