Offhand, my educated guess based on how much it costs /me/ to drive, is that for any form of transport the order of costs from greatest to least is:
1) The human driver.
2) The fuel.
3) Maintenance.
Even at a minimum wage, what really breaks bus routes is the human driver. You could send a smaller (more fuel efficient for low loads) vehicle on less profitable routes, but that doesn't modulate the biggest cost at all.
Google, Ubur, Lyft, all seem to see a future where (to borrow from the older Total Recall movie) Johnny Cab is able to eliminate a lot of public transit and ride-sharing issues.
I believe that the technical challenge is within our grasp (it'll be difficult but possible).
It might, with sufficient volume, be cost effective and when factoring in the difference in accident rates surely will be.
The hardest sell will be the truly glacial speed of modifying laws to get these vehicles on the road without drivers.