If booking an airline flight seems to cost more than it did just a few months ago, it's not your imagination -- many airlines have already raised prices three times this year.

In the most recent bump, United and Continental increased the cost of round-trip tickets between $4 and $10, and Virgin America, Delta Air Lines, US Airways and American Airlines mirrored the move and raised their fares as well (JetBlue Airways and Southwest Airlines haven't yet followed suit).

So what's behind the jump?

Airlines for America, the trade group for the nation’s largest airlines, says while fuel-efficient jets have allowed airlines to cut fuel consumption, the 40 percent rise in gas prices between 2005 and 2011 negated most of the savings -- resulting in what airlines say is a necessary price hike.

Still, the nation’s airlines ended 2011 with a 0.3 percent profit margin, largely because of cost-cutting measures like staff reduction and facility consolidation. Filling more seats on every flight has also helped, and all those fees we hate so much -- like bag-check fees and charges for on-board food and entertainment -- allowed airlines to make an average of $22 more from each passenger's round-trip fare.

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