In the UK you would need ~5-6 drivers to run a vehicle 24/7 accounting for breaks, holidays, illness etc. If you pay them £10.50 (about the London bus driver rate) then after pensions, national insurance etc you're talking around £3,000 a week, which is £160,000 a year (around $230,000). The driver and the equipment needed for a driver take up space and add weight that adds more additional cost.
London buses are expected to have a £350,000 cost over their 14 year lifetime, while based on the above a drivers costs £2.24 million over 14 years (I'm assuming higher utilisation though). In short, if self-driving tech doubled the bus purchase and running cost it would still cost £1.9 million less over its 14 years in service.