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.
The current toll to cross the bay bridge by car from Oakland to sf is $8, and like someone mentioned it’s only $4.25 from Oakland to sf without the airport charge, so you are still saving by using bart, just not as much as you probably should.