Before Southwest Airlines said yesterday that it would start flying between Boston and Philadelphia in June, air travelers on the route didn't have a lot of choices - or cheap tickets. The only airline that flies nonstop from Logan International Airport to the City of Brotherly Love is US Airways, which on its website yesterday was charging $1,100 round-trip, with advance purchase, on its 15 daily, nonstop departures. But the low-cost carrier's entrance on the route, with five daily flights and introductory fares starting at $118 round-trip, is already shaking up the market. US Airways filed a $59 one-way sale fare yesterday, beginning June 27, the day Southwest starts flying at the same price. 'You won't see it that low again,' said Rick Seaney of Farecompare.com. 'That's what we call introductory predatory pricing.' US Airways did not offer details on the $59 fare, but spokesman Morgan Durrant said the carrier's regular Boston-Philadelphia ticket costs are justified. 'We've priced what the market will bear and we'll continue to do so,' he said. Southwest began service from Boston in August and serves four destinations out of Logan. The Dallas carrier received a lot of requests from people to add Philadelphia because of the ticket prices, said spokesman Paul Flaningan. 'They preferred to drive from Boston to Philadelphia rather than flying because it was so expensive,' he said. When AirTran Airways stopped flying the route in late 2007, prices more than doubled - from $152 to $344 for the cheapest round-trip ticket - and continued to climb after Delta Air Lines pulled out last August, to $462. Since then, US Airways has been the sole carrier flying nonstop between the two cities. The majority of passengers flying from Boston to Philadelphia are business travelers, who like the hourly shuttle and don't squawk about paying a premium for it, said Daniel Kasper, head of the transportation practice at the Cambridge office of LECG, an economic and financial consulting firm. 'Business travelers are less price sensitive and more schedule sensitive,' he said. 'They can fill their planes with business travelers and charge higher fares.' Southwest hasn't set its schedule yet, but Flaningan said the airline planned to fly during peak times to accommodate business travelers. The airline, which doesn't have reserved seating, has been trying to attract more business travelers and recently started allowing advance boarding for a fee. All the low-cost carriers are going after business travel now, said Edward Freni, director of aviation at Logan, especially as companies look for ways to save money during the recession. And Southwest's Boston-Philadelphia debut will help adjust costs for business and leisure travelers alike. 'What I think this is going to do is right-price that market,' said Freni, noting that the route was still ripe for competition. Carly Kuper, director of communications for Digitas Health in Philadelphia, travels to the interactive marketing agency's headquarters in Boston every quarter. 'Usually I travel by train because it is so expensive and sometimes inconvenient to fly,' she said. But a cheaper plane ticket might entice her to visit Boston for nonbusiness reasons. '$59 each way might have me traveling more frequently to go to Mike's Pastry,' she said. Katie Johnston Chase can be reached at johnstonchase@globe.com. 