NEW YORK - To fly someone from New York to Los Angeles and back, airlines spend close to $330 these days - just on fuel.

That's a 48 percent increase from last year and the main reason vacationers face record costs to fly this summer. To offset their single biggest expense, airlines have hiked fares seven times this year and raised fees for checking bags and other services.

This has only added to the frustration of most casual fliers who see $59 fares advertised but are quoted prices well above $300 when they actually try to book. Americans' expectations of a cheap vacation are being destroyed by the reality of $100-a-barrel oil.

"The passenger has to understand that the airline industry in the United States is not meant to be a low-cost mass transit system. The airlines are in business to be profitable," says airline analyst Robert Herbst.

A decade ago, fuel accounted for about 15 percent of airline operating expenses. Five years ago, it was 29 percent. Today, it's 35 percent.

During the first three months of 2011, the airlines spent $8.7 billion on fuel, 31 percent more than last year. In the current quarter, jet fuel expenses are even higher.

U.S. airlines burn an average of 22 gallons of fuel for every 1,000 miles each passenger flies. At $3.03 a gallon, airlines are currently spending $330 per passenger just on fuel for a 4,950-mile transcontinental round-trip. Some fliers might have paid less than that for their ticket while others could have spent more than $2,000.