The airport has worked hard to keep costs as low as possible, said Mark Reis, the Port of Seattle's managing director for Sea-Tac. And Southwest's operating efficiencies could make its average cost much lower than that $15 figure, he said.
The passenger cost at Sea-Tac has prevented Southwest from offering cheap, short-haul flights from the airport, such as flights to nearby Portland, Ore., Ms. McInnis said. Southwest couldn't make a profit from such flights unless it cut costs, she said.
Southwest is evaluating how much its costs could go down if it moved to Boeing Field, Ms. McInnis said.
"If we did not think the cost savings would be dramatic, we would not have undertaken this step," she said.
Sea-Tac's new runway was designed to address a serious issue that often slowed traffic to the airport. Its two existing runways are so close together that air traffic controllers have to route planes in a single-file line when there are low clouds and limited visibility.
The Puget Sound Regional Council, which represents four counties, recommended the third runway in 1996 after evaluating the idea of expanding secondary airports to accommodate the area's growing travel needs.
"We, as a region, need to ensure we make the most effective use of the investment that has been made," said Mr. Reis of Sea-Tac.
While American tries to convince Southwest to move to D/FW, Seattle-based Alaska Airlines has called on Southwest to stay put at Sea-Tac. Like American, Alaska has said it will move some flights to the smaller airport if Southwest gets a competitive advantage there.
Some of Love Field's residential neighbors have worried about the noise and environmental impact caused by additional flights near their homes.
Mr. Burke of Boeing Field said he's already hearing from neighbors about Southwest. If it gets a proposal from Southwest, Boeing Field will study how the airline would affect the neighborhoods, including the impact of noise, air quality and auto traffic, he said.
Alan Rothblatt lives about a mile from Boeing Field. Last week, the physician was a passenger on one of the small airport's three passenger airlines, which make short trips in tiny planes to the San Juan Islands and other off-the-beaten-path destinations.