U.S. natural gas prices are finally reaching a safe zone. At near $4.50 (U.S.) per million British thermal units (Btu), drillers can turn a profit on gas after several painful years. Cold weather has helped push up the price, but with demand rising it is more than a blip. Meanwhile, production costs have come down. The likes of Exxon Mobil and Chesapeake Energy can breathe easier.