I agree that ultimately privately owned airfields generally have the right to charge whatever they choose. On the other hand, it is irksome when the charges are quite so high. If you plan to go somewhere, as oppose to just fly to get a burger for fun, then you don't always have the discretion to fly to a different airfield.
I flew from Blackbushe for a little while, including during weekdays, and there was very little conflict between bizjets / commercial traffic and the flying school aircraft, at least as far as I could tell. Likewise at Inverness, where it was generally straightforward to stay out of the way whilst the bigger aircraft came in and out.
There does seem to be a trend that even smaller airports are becoming prohibitively expensive, and from the outside I agree that it's hard to imagine that anyone will be profiting from this.