SYD "AirportLink" (Train) has a standard one-way adult fare of $11. There are also OFF PEAK (after 9am) Day return fares for $14.30. There are several other discount fares as well, more here:
SYD AirportLink Fares.
One interesting one is the
Group fare where up to 4 can travel for somewhat less than the normal cost of a shared taxi.
MEL Skybus standard fare is $13, you can get a return for $24. That's about the only adult discount, more
HERE.
BNE Airtrain has some interesting discounts. Aside from pre-purchasing vouchers for $8 each in books of 10 or more, there are others for group booking purchased online. More
information here.
Mal, can you get $8 fares at the airport stations?