King County Metro Transit Hybrid Articulated Buses: Interim Evaluation Results[1] (2006) page #24 (PDF page 35/58) shows a table of several individual buses of different types, with mileage, parts cost, and labor hours, summed into cost-per-mile. Diesel had an average cost per mile of $0.48/mile. Hybrid buses kept at one bus base had a cost per mile of $0.47, while Hybrid buses stored at another base had a cost per mile of $0.46.
I'm unsure about the national averages - they seem to be twice the Seattle rate. And KC Metro's electric buses are estimated to have less than half of the maintenance costs of other buses. And the national average for hybrid is higher than diesel, while KC Metro's hybrid buses have slightly lower maintenance costs than their diesel. Unfortunately, I couldn't find numbers on KC Metro's electric trolleys or CNG buses.
By comparison, the 2016 IRS rate for Business Mileage is $0.54/mile. [2]
Operator Type Average Maintenance Costs Per Mile ($)
KC Metro Electric 0.20 (2016) (estimated)
National Diesel 0.90 (2016)
KC Metro Diesel 0.48 (2006)
National Hybrid 1.10 (2016)
KC Metro Hybrid 0.47 (2006)
National CNG 1.00 (2016)
IRS Mileage Rate 0.54 (2016) (car)
[1] http://insideevs.com/proterra-catalyst-electric-bus-complete...
Wikipedia just has an unsourced "Electric motors typically last longer than internal combustion motors, and cause less secondary damage from vibration, so electric buses tend to be very long-lived as compared to motorbuses"