When new housing facilities, as provided in this chapter, have been completed, the governing body shall fix the maximum rental per room that may be charged by the applicant to the tenants of the new housing facilities, upon such a scale that the net return to the insurance company from the rentals so fixed shall equal at least five per cent but not more than six per cent upon the cost of the entire project after payment of all items set forth in paragraph “d” of section 17:19-2 of this title. From time to time the governing body may increase the maximum rental per room so fixed, upon satisfactory evidence presented by the insurance company of such increase in taxes, insurance or costs of operation or maintenance as requires the increased rental to assure a return of at least five per cent upon the cost of the project.