The airline industry, led by Delta Air Lines ( DAL) Friday, made another attempt to boost ticket prices to offset the high price of fuel. But with low-cost carriers rolling out new service while legacy carriers increase the number of flights, a fare war undermines the industry's ability to increase fares. Delta boosted the price of a round-trip ticket by $8 to $9 in markets with a connecting flight. By Friday afternoon, Northwest Airlines ( NWAC) announced that it was matching Delta in select markets where the two carriers compete. But in order for the fare hike to stick, rivals must all join in -- something that hasn't happened since the World Trade Center attacks 2 1/2 years ago. Three weeks ago, Continental Airlines ( CAL) attempted to boost round-trip tickets by $10, with the company complaining oil prices were "the highest we've ever seen." But while most of the legacy carriers matched Continental's hike, including Delta and Northwest in select markets, low-cost carriers didn't join in and the hike was rescinded. A decade ago, airlines had more pricing power, especially at the high end dominated by the legacy carriers, which could always count on the loyal and lucrative business traveler. But with the Internet exposing the four-figure pricing gap between business class and coach class and low-cost carriers adding more flights, studies show that the business travel has moved away from the legacy carriers, taking pricing premiums with them. "In past airline industry cyclical downturns, many business travelers traded down to low-fare airlines and quickly traded back up to full-fare major airlines with the first signs of economic recovery," said Kevin Mitchell, spokesman for the Business Travel Coalition, a trade group representing business travelers. "Just like the workers that are not replaced when an industry changes structurally, business travelers are never again going to accept sky-high business fares."