The payment to United Airlines is emerging as a key issue in a fight between Southwest and Delta Air Lines, which is trying to keep a toehold at the Southwest-dominated airport. Southwest counters that selling gate rights is not new — American Airlines did so when it had to divest assets to win approval for its merger with US Airways. Assuming that Southwest operates about 10 flights per day at each gate, he said, the price was less than Southwest paid for takeoff and landing rights at LaGuardia Airport in New York and Reagan National outside Washington, D.C.