Taking train to city is indeed cheaper than taxi/uber. Note airport charges around $18 to come in and going away from domestic/international airport then normal charge apply. To save this, it's cheaper to take bus to and from Mascot station (1 station away) then take train normally.
Before you come here, park for free in nearby Wolli Creek park for 3 hours free to wait for the passengers to be in that 15 min free car park, then once they're there, you start driving away to pickup.
