For an organisation such as Finningley/Waddington/Leuchars they usually held back a portion of takings as nest eggs as you say. For a new venue, Waddington/Coningsby the issue was to raise as much revenue for charity through minimizing costs and creative accounting. Insurance, if any, was minimal.
Fuel was often diverted. We had a P51 that arrived with empty drops, refuelled, did a display somewhere else, returned, dropped the tanks, displayed, and then refitted tanks and departed full.
I refused John Watt's Catalina for the same reason - arrive empty, depart full.
My staish also flatly refused to pay Binbrook for T&S, the Reds and Falcons for smokes and refused free entry to their families. TB!
As an occasional airshow performer (not in a P-51 unfortunately), with the best will in the world, you will not get the "acts" unless you pay for them. A WW2 fighter costs around £3500 per hour to operate so it will require some (a lot of) fuel to tempt it to your show with likely a commensurate fixed rate. I charge a different rate depending on whether the organiser includes fuel or not. Overall, I break even. The organiser may benefit if he is able to get fuel cheaper than I can.