No metropolitan utilities district may extend or enlarge its service area unless it is economically feasible to do so. In determining whether or not to extend or enlarge its service area, the district shall take into account the cost of such extension or enlargement to its existing ratepayers.

All books, records, vouchers, papers, contracts, or other data indicating the economic feasibility of such extension or enlargement shall be filed with the secretary of the board of directors of the district and shall be open to public inspection.