The IRS reimbursement rate for auto expenses for business is about $0.50/mi. Uber rates (likely subsidized) are in the $1 - $1.50 range. You can argue about the details but your floor for self-driving cars is around what the cost to operate a vehicle is. Fleet operations have some economies of scale but they also have management and staffing costs.

Electric vehicles may someday bring down costs across the board.

Bottom line is that autonomous driving isn't a path to "too cheap to meter" transportation especially as it increases congestion in various ways.