Replacing the entire fleet of cars with self-driving cars isn't cheap either; so both require a significant investment. So does owning a car for that matter: both in direct car ownership, but also public cost in road maintenance, health care costs, parking lots, etc.
With fewer cars it's a lot easier and cheaper to get quicker and better public transit going. I don't know anything about Portland, but in general going by bus anywhere is mostly waiting in traffic (i.e. cars) instead of actually driving. This is why some places have bus lanes, which do help, but in a limited fashion.
I don't know how it compares exactly, but it's quite complex and there are a lot of factors and I'm not so sure public transport ends up being significantly more expensive in the end.
With fewer cars it's a lot easier and cheaper to get quicker and better public transit going. I don't know anything about Portland, but in general going by bus anywhere is mostly waiting in traffic (i.e. cars) instead of actually driving. This is why some places have bus lanes, which do help, but in a limited fashion.
I don't know how it compares exactly, but it's quite complex and there are a lot of factors and I'm not so sure public transport ends up being significantly more expensive in the end.