For aid distributed in school fiscal years prior to 2015-16, excess cost means the difference between the total cost of the special education program excluding residential care and the number of students in the special education program multiplied by the adjusted average per pupil cost of the preceding year for the school district of residence of each child. For aid distributed in school fiscal year 2015-16 and each school fiscal year thereafter, excess cost means the difference between the total cost of the special education program excluding residential care minus federal medicaid funds received pursuant to section 43-2511 for services to school-age children excluding amounts designated as reimbursement for costs associated with the implementation and administration of the billing system pursuant to section 43-2511 and minus the product of the number of students in the special education program multiplied by the adjusted average per pupil cost of the preceding year for the school district of residence of each child.