Section 15. The director of accounts in the department of corporations and taxation shall cause an audit to be made annually of the accounts of all districts organized under the authority of section ten and for such purpose he, and his duly accredited agents, shall have access to all necessary papers, books and records. The expenses incurred for said audits shall be paid primarily by the commonwealth. Said director shall apportion the cost of each audit among the several municipalities comprising the district on the basis of the taxable valuation of said municipalities as last established by the general court for state and county taxes, and submit the amounts of each apportionment to the state treasurer, who shall issue his warrant requiring the assessors of the cities and towns which comprise the district to assess a tax to the amount so apportioned, and such amount shall be collected and paid to the state treasurer as provided by section twenty of chapter fifty-nine.