When doing the costings you have to weigh up the fixed and variable costs and divide them by the hours as accurately as you can predict baed on past costs. If an aeroplane is hired on tacho then the hourly rate will be worked on 100 hours flown per 100 hourly. If on hobbs, more like 115 hours per 100 hourly. So you can hire on tacho, but it will cost 15% more, for example.
If you think you are being robbed then best to discuss this before the flight than after.