You can allow prorated billing so that when registrants purchase a product in the middle of the billing cycle, they pay the prorated price rather than the full price.
Proration algorithm is based on the number of days in the season. Registration price is prorated daily if the registration happens after the start of the program. For example, if there are 100 days in a season, a registrant purchases the product 10 days later after the season started. The prorated price is (1-10/100)=90% of the full price.
If you set up family discounts or discount coupons, the discount will be prorated. For example, if the prorated price is 90% of the full price, the discount will be 90% of the full discount price.