Many of us are heading overseas and there is a standard PCR test requirement within 72 hours with proper test results instead of just SMS, for most airlines.
For Sydney, the cheapest options is via Histopath @ $100, done at airport on the previous day (considering your flight is after noon as the results will be available only at 9 am next day).
Next cheapest is 4cyte at $120
Looks like there are other locations which offers $100 tests.
* Granville as per @DaddyJi