With a big management company, or even some FSPs, you can get some leverage and demand good service and reasonable charges. Otherwise, it can be quite difficult for single aircraft.
I wonder if it might be possible to have some sort of TripAdvisor / Yelp-style thing for FBOs? In the US there is the highly informative AirNav with FBO ratings and comments, but it seems to be sorely lacking in Europe.
Even if there is a monopoly and you have no other choice, at least you / your pax would know what to expect and could prepare accordingly.