The first major cost is the track. Each facility has different rental fees, and then you have to understand what comes with it, and what doesn’t. Certain facilities may need you to bring in security, or a front gate person to sell the pit passes. You may need to source out your own Ambulance or medical personal.
Pit pass, parking, entry fee and practice fee all go towards that initial cost. The money left over goes to paying the staff hired to put on the event. That money left over is to pay for prizes and other items.
There is a long list of costs that the normal racer does not see when someone or a group is putting on an event. It takes a lot of work and time, and is not something you can just pick up and do at any moment.
From experience in Road Racing here in Michigan, I believe our break-even point was around 250 entries, depending on the year. If we had less, the club lost money. If we had more, we made money to be able to buy year-end trophies and save for a rainy day.