The necessary administrative and overhead expenditures as determined by the board of the regional educational service center shall be shared jointly by the participating boards of education. In addition any participating board of education and nonpublic school shall be required to pay a prorated share of the costs of any program or service to which it subscribes. Any commitment made by a participating board of education or nonpublic school with a board of a regional educational service center in accordance with any provision of this part shall constitute a valid obligation within its appropriated or other available funds.