As it turns out it doesn’t actually cost that much on regular transit, there’s an AIRPORT SURCHARGE because it’s an “airport train”.
No wonder Americans don’t use public transit, even when the system exists it’s ridiculously difficult and expensive to use.
But in Sydney you will pay the surcharge only when you get off or on at the actual airport station. Just using that line and passing the airport will cost you nothing extra. Usually less than 4 AUD for the whole trip.
It’s the same situation with BART. The surcharge only applies when using the airport stations. No extra charge if you’re passing by.