Having owned aircraft since 1974, anyone who thinks they are going to make money from an aircraft on line be it a Chipmunk or a Citation Jet is deluding themselves (I've owned both)
Similarly with overheads charged out properly by the flying school sub leasing an aircraft is not a winning formula.
It is simply supply and demand.
Private owners struggle to get utilisation, where's flying schools have a ready market thru their students, be it limited.
If you need to get hours to afford the aircraft youly probably better off not owning. If you are using it as a tax reduction vehicle then so long as the rate is not ridiculously low then just accept that the school needs to make money, after all you are both "in business" and you agreed the rate by mutual consent???
As for getting upset at what they charge, that is their business NOT YOURS