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 95 percent of the purchase price and the FreddieMac 30-Year Fixed Rate- Mortgage rate of interest), property taxes, homeowner and private mortgage insurance and condominium or homeowner association fees do not exceed 30 percent of the eligible monthly income of the appropriate size household as determined pursuant to N.J.A.C. 5:80-26.7, as may be amended and supplemented; provided, however, that the price shall be subject to the affordability average requirement of N.J.A.C. 5:80-26.4, as may be amended and supplemented.