That's actually a very difficult question to answer and depends on a huge number of factors. Taking a couple of extreme scenarios;
Fred, who is 76, gets paid £5.60/hour working for Claptous Buses driving a life expired Bristol VR with no asset value and bald tyres. This type of service costs not much more than the diesel used, wages and insurance to provide and is probably more common in rural or semi-rural areas. When not on school runs, the bus either sits idle (if it hasn't broken down again) or runs the granny shopper service jointly funded by Nowhereshire county council and Bigshopco.
on the other hand,
Vladimir, who is 29, gets paid £13.60/hour (he's on overtime) working for FirstStage Buses driving a nearly new £150,000+ bus. This meets the latest environmental regulations but whose asset value needs to be written off after 10 years. This bus is well maintained but not especially economical to run (being heavy to meet crash regs and having air con). When not on the school run, the bus must find employment on commercial services to cover the depreciation, insurance and support costs.
relative costs of these types of service...haven't got a clue myself; just illustrating how difficult it is to answer questions like this.