Ask HN: How should I price a seasonal/temporary service?

I have a system which helps event management companies to handle the registration for events, take payments, handle scheduling, task workflow, configuration etc. It has been in use for several years for some of my customers who I engaged with and sold directly, but now I want to pivot the sales model and put it out for public consumption on the web as an available service.

One challenge for me right now is around pricing strategy. My target is a group/person who hosts and sporting tournament, baseball for example. Typically these happen over a long weekend and the ramp up ahead of the event is several months. I could price it on a monthly fee but the baseball tournament has a peak cycle on the calendar, if the tournament is in October then registrations probably start in late July or August. November until June is dead time and the customer does not want to pay for the service that is doing nothing for them during that period.

I could go with a participant cost model, maybe $2 per baseball player but that ignores the fact that there is a vast difference between a single elimination football tournament where each team has as many as 100 players and a golf tournament which has only 100 people participating all together.

My other thought is to a percentage spend model, since we handle your payments via credit card or ACH we just up the percentage charge to say 5%, kick the processing fee back to the processor and keep the spread. This could work but lots of the organizations who participate in organized competitive events only have checking accounts and prefer to pay via paper check.

Any ideas on some novel pricing approaches?