The net purchase price (i.e., gross sales prices minus subsidies) paid for the unit by the selling owner, increased by the percentage increase, if any, in the Consumer Price Index for Urban Wage Earners and Clerical Workers in the New York-Northern New Jersey Area, as published by the united States Bureau of Labor Statistics (the "Index") on any date between a) the month that was two months earlier than the date on which the seller acquired the unit and b) the month that is two months earlier than the month in which the seller contracts to sell the unit. If the Bureau stops publishing this index, and fails to designate a successor index, the municipality will designate a substitute index; and