This fall, 75 clean-diesel buses purchased last year will replace older buses in the fleet of nearly 730. Those additions will reduce maintenance costs and emissions, potentially by up to 90 percent, McLean said. The Port Authority has an option to buy another 400 clean-diesel buses over the next five years. Each costs $455,415 and is paid for by the federal government as long as it remains in service for 12 years or 500,000 miles, McLean said.