This stuff varies a lot by location. Sure in San Francisco and a handful of other cities parking and tolls cost a great deal, but that's not the case in most locations and presumably Waymo's goal is to expand far beyond a few locations. People in, say, Phoenix aren't paying for parking. If you don't have a car payment your monthly transport costs are going to be much lower than that.

Add tag tax, residential parking, subsidized work parking, maintenance, incurred violations, tolls.

400/mo or 5000/yr for not having to worry about all that plus never playing the "wait let's circle the block, maybe a spot has opened up" game... sounds tempting.