For the price of extending the Big Dig to Dorchester, humans could possibly dig at the red soils of Mars, see if anything wriggles in underground reservoirs and potentially resolve the question of whether life exists there. “I believe if you had two to three times the price of the Big Dig, you could have humans on Mars,” said John Mankins, manager of NASA’s program in human exploration and development of space. That’s between $28 billion and $42 billion.