You have to pay the driver, benefits, maintenance of stops, and subsidize all the miles where the bus is below capacity.

And we have to pay the shareholders of Connexxion, Qbuzz, Keolis and Arriva.

This!

Let's assume a driver's salary of 18 €/h. A rule of thumb I've heard is that vacation, sick days, social security etc. etc. approximately double that cost to the company, so 36 €/h. 20 kph average speed is already a somewhat generous value for a regular non-BRT city bus line, but let's use it for simplicity's sake, which gives 1,80 €/km driver's costs.

Another rough value of thumb for city buses is that they use roughly about 30 l/100 km, which adds about 0,50 €/km, so 2,30 €/km for driver + fuel.

A regular city bus holds about 100 passengers, and an average utilisation figure as averaged across the whole day and all routes might be somewhere around 20 %. So an average of 20 passengers per bus at all times, which means 11.5 cents per kilometre for driver and fuel. Now add-in capital cost for the bus itself, maintenance and other ancillary stuff and unfortunately 25 cents per kilometre isn't all that much all of a sudden.

(If you can read some German, somebody did a more complete modelling of costs at https://prof.bht-berlin.de/fileadmin/prof/jschlaich/200811_F..., and with some assumed fictitious but presumably plausible values ended up at 3,26 €/vehicle-km for a city bus operation in 2008 – and both (driver) pay and fuel have increased above inflation since then.)