Same problem as I used to have with Jeep events. People would register and not attend, depriving others from atttending, or far worse not turn up leaving the club to foot a huge bill where fixed site costs had to be paid. You can't simply increase the price to those who did attend...
As for price Middleton Hall that run an event to generate restoration funds for the hall restoration pitch at £4 per car. They have the same of drinks, burgers on site and nice grounds to park in, volunteers, etc. They attract plenty, as you know