(a) The resale price of shares in a mutual company shall be fixed by the mutual company, subject to the approval of the commissioner or supervising agency and shall be equal to (1) the consideration the selling tenant-cooperator paid for such shares and (2) any capital assessments and voluntary capital contributions approved by the commissioner or supervising agency and paid by the selling tenant-cooperator to the mutual company, to the extent not already included in the consideration paid for such shares, and, if established by the mutual company, (3) a proportionate share of the actual aggregate amortization paid on all existing and prior mortgages on the project in reduction of total outstanding principal indebtedness during such period as shall be fixed by the board of directors of the mutual company, to the extent not already included in the consideration paid for such shares, and (4) reasonable administrative charges.

(b) The aggregate amount to be paid to the selling tenant-cooperator with respect to the sale of the selling tenant-cooperator's shares shall be fixed by the board of directors of the mutual company, subject to the approval of the commissioner or supervising agency, and shall be equal to (1) the consideration the selling tenant-cooperator paid for such shares, (2) any capital assessments and voluntary capital contributions approved by the commissioner or supervising agency and paid by the selling tenant-cooperator to the mutual company, to the extent not already included in the consideration paid for such shares, and (3) a proportionate share of the actual aggregate amortization paid by the selling tenant-cooperator on all existing and prior mortgages on the project in reduction of total outstanding principal indebtedness during such period as shall be fixed by the board of directors pursuant to subdivision (a) of this section, to the extent not already included in the consideration paid for such shares. To the extent that a selling tenant-cooperator may be entitled to an amount less than the resale price of his shares, the difference shall be retained by the mutual company.

(c) The board of directors may, subject to the approval of the commissioner or supervising agency, establish a general policy pursuant to which a selling tenant-cooperator who had occupied more than one dwelling unit is paid an amount measured by his proportionate share of the actual aggregate amortization paid during his period of occupancy on all existing or prior mortgages on the project. To the extent that a selling tenant-cooperator may be entitled to an amount greater than the resale price of shares, the difference may be paid to the selling tenant-cooperator by the mutual company.

(d) The "proportionate share of the actual aggregate amortization paid on all existing and prior mortgages on the project" referred to in subdivision (a) of this section shall be in the same ratio to such actual aggregate amortization as the number of shares held by the selling tenant-cooperator at the time of sale bears to the total number of shares of issued and outstanding capital stock of the mutual company during such period.

(e) Nothing contained in this section shall prohibit the continued use of any method of calculating resale price adopted by a mutual company and approved by the commissioner or supervising agency prior to the effective date of this section.