The numbers are pretty easy. Secondhand C146a Garmin 430's are about $6,500 ea. New GTN 650's are about $12k. Plus, you probably need a new grey code altitude encoder. That's maybe $500. Plus you need new co-ax cable because WAAS requires a different impedence cable. Call that $100. And an ADS-B transponder - circa $3,000.
Depending on whether you use new GPS boxes or old, there is $17k to $28k of hardware.
You will then probably require some uniquely Australian / uniquely CASA engineering orders for the installation. Probably add another $1k.
Then finally is the install, which depending on panel requires a radio stack relay, some disassembly of the interior to install new cable and installation of new antennae. I'm guessing, but I reckon that's a minimum of 2 days labour - maybe 5. So the installation is going to be $3K to $5k.
There is no real volume pricing benefit.
All of this for an airframe that is likely to be worth $100k. I can see a lot of guys quitting IFR. Which is probably CASA's real objective.
If we GA had any sort of robust representation we would be lobbying for engineering orders to be not required for routine non-structural work like fitting radios and for GST to be waived on equipment required to upgrade to PBN.