BOSTON -- You've probably seen the ads offering some very good deals from the nation's big airlines.

Boston to Los Angeles for $98 dollars one way. Boston to JFK for $64. How about Boston to Shanghai for $315?

Some are too good to be true, Team 5 Investigates reported Monday.

"These fares are 'come on fares,'" said Bob Weiss, an expert on the travel industry. "They're very tightly controlled by computers by the airlines."

The cheap seats are real, but they are hard to get.

"The stars need to align and your computer needs to align because that's the key to it," Weiss said.

The airlines offer the lowest fares for only a few seats per plane. You have to be willing to fly certain times of the day and days of the week.

For example, Team 5 Investigates saw an ad for Boston to Los Angeles, only $98 one way. You can only fly Tuesday, Wednesday or Saturday --- and even then, there was only one option departing May 12th and only one back the following week.

Jet Blue offers an $89 deal each way from JFK in New York to Ft. Lauderdale, but only on very few flights.

Fares are more expensive when booking by phone.

Team 5 Investigates' Susan Wornick called JetBlue about an ad offering $71 one way from Boston to Washington-Dulles.

"$149, and that's just one way," Wornick was told by customer service representative. Wornick was told the $71 fare was just for certain dates.

Another ad offered a ticket from Boston to Puerto Vallarta for $149 one way on Continental. Wornick tried to book that round trip.

"$1,059?," Wornick asked. "I'm actually looking at an ad that says $149," she told the representative.

But again, the Continental representative explained the offer only applied to a few seats on a few dates.

Weiss said the deals are offered mostly to get your attention.

"They realize if they get you into the computer system, primarily you'll search for the lowest fare and say, well, I can't get this fare, so I'll settle for something that's $20 or $30 more," Weiss said. "Once they've got you on the computer, they feel they've got you hooked."