As has been pointed out in this and other threads, the cost of installing ADSB equipment is falling exponentially as we get closer to the USA 2020.
To nitpick - the cost of INSTALLING ADS-B equipment in Australia is not falling. The cost of the ADS-B equipment might be falling, but the major cost is the installation itself.
The FAA in the US allows ADS-B installations to be done under "Field Approval". CASA however still require the full deal including engineering orders for the installation.