The initial purchase price for all restricted ownership units shall be calculated so that the monthly carrying cost of the unit, including principal and interest (based on a mortgage loan equal to ninety-five (95%) percent of the purchase price and the Federal Reserve H.15 rate of interest), taxes, homeowner and private mortgage insurance and condominium or homeowner association fees do not exceed twenty-eight (28%) percent of the eligible monthly income of the appropriate size household as determined under N.J.A.C. 5:80-26.4, as may be amended and supplemented; provided, however, that the price shall be subject to the affordability average requirement of N.J.A.C. 5:80-26.3, as may be amended and supplemented.