The rate at which the sums to be so charged as provided in Section 6 of this Act shall be calculated by the board of directors is the average per capita operating cost for all persons receiving the benefit of such facilities or services computed for each fiscal year; provided, that the board may, in its discretion, set the rate at a lesser amount than such average per capita cost. Less amounts may be accepted by the board when conditions warrant such action or when money is offered by persons not liable under Section 6. Any money received pursuant to this Section shall be paid into the governmental unit’s Fund for Persons with a Developmental Disability.