The director shall fix the maximum rental or charges per room to be charged the tenants of the housing accommodations furnished by any limited dividend housing corporation. Such maximum rental or charges shall be determined upon the basis of the actual final cost of the project containing such rooms so as to secure, together with all other income of the corporation, a sufficient income to meet all necessary payments to be made by the corporation as herein prescribed and such room rental rates or charges shall be subject to revision by the director from time to time. Letting, subletting or assignment of leases of apartments in such housing accommodations at greater rentals than prescribed by the order of the director are prohibited and all such leases will be void for all purposes.