The offers can be tempting: buying airline miles to boost your mileage account. But how do you know you're getting a good deal and not just adding to the airlines' bottom line?

Most airlines now offer frequent flier miles for sale, but experts say consumers should think carefully before clicking the buy-now button.

If you're a frequent flier, you're probably used to just racking up points effortlessly as you whip out your rewards credit card, shop at affiliated retailers and book flights. If you come up a few miles shy of a dream vacation, though, you might wonder whether these dollars-for-miles offers are a smart move -- or just a tricky marketing ploy. Experts say airlines usually get the better end of the deal, but savvy travelers can come out on top.

The basics of buying miles
How much is a frequent flier mile worth? That's the basic question you need to ask when considering purchasing miles, experts say.

"The price airlines charge typically is about 3 cents a mile when everything is factored in -- but, these days, the average value of a mile is about 1.2 cents," says Tim Winship, publisher of FrequentFlier.com, which offers tips and information about frequent flier programs. "Unless you go into it knowing in advance precisely how you're going to get at least 3 cents in value out of each mile, you're overpaying."

However, experts say paying too much per mile can still work in your favor if you need only a small number of miles to reach an expensive award ticket. "If you need, say, only 1,000 more miles to make a trip and you want to spend $27 to get it -- well, you need it. That's fine," says Chris Lopinto, president and co-founder of ExpertFlyer.com, a frequent flier information service.

In general, though, experts say selling frequent flier miles is a way for airlines to cash in while spending very little in return. "The sale of miles is extremely profitable for the airlines," Winship says.

For example, a consumer could spend about $750 to purchase the 25,000 points typically needed for the lowest-level award, a restricted coach-class domestic ticket -- a ticket that, on average, would cost about $350 to buy.

On top of that, Winship says, most frequent flier seats are seats that would not have sold, so they would have flown empty anyway, making selling miles at inflated prices even more lucrative for air carriers. Winship says: "What does it cost to fly one more passenger? A little bit of extra jet fuel, a can of Coke and some peanuts."

Tips and tricks for buying miles
To make sure the purchase of miles helps pad your wallet -- and not the airline's bottom line -- experts offer these tips:

First, do the math. Look at how much the miles will cost -- including taxes and fees -- and compare it to the cost of buying a ticket outright and saving your miles for later. "Usually, if it's an expensive trip and if you're close to having enough miles -- close might mean less than 10,000 to 15,000 miles -- buying miles might work out a lot better," says Gary Leff, co-founder of the frequent flier community MilePoint.com. "It usually works out well for international tickets, especially in business class or first class," he says. For example, Leff recently helped a traveler who needed two business-class tickets to Kenya. "They were about 16,000 miles short, and spending the roughly $500 to buy the miles made sense because it would have cost $8,000 or $9,000 to buy the tickets out of pocket," Leff says. "That's one example where it worked wonderfully."

Make sure award seats are available. "I would reserve the award seats and then go buy the miles," says Marybeth Bond, travel expert, author and founder of GutsyTraveler.com, who recently purchased 10,000 miles so she could take her two daughters on a vacation to Thailand. "Usually, you can make the reservation and then you have at least 72 hours" to use your miles to pay for the tickets. However, experts say you should check with the airline first, as some don't allow you to hold a seat if you don't have the required number of miles in your account and others require you to reserve by phone in order to hold a seat. Also keep in mind that there can be a lag in posting purchased miles. Some airlines say to allow 72 hours for them to appear in your account.

Consider alternatives to buying miles. Before you buy, look at whether there are other ways to earn the miles you need, experts recommend. "There are so many ways to earn miles -- that's something people need to keep in mind," Winship says. "All of the big [frequent flier] programs have mileage malls -- extensive networks of online retailers that give members miles when they make purchases online. You're short of miles and you need a pair of khakis? Well, go to Gap.com and earn some miles. Or maybe start in on your Christmas shopping a few months early. Some people are overlooking this, and it's potentially a pretty rich source of miles."

Use your rewards credit card to buy the miles. That way, you can earn even more miles -- or other rewards -- for your purchase. "You obviously should use that card to purchase miles. It makes it a slightly better value proposition for you," Winship says.

Special offers: dubious bargain?
Most airlines run promotions several times a year, offering additional bonus miles with the purchase of a certain number of miles. These offers can seem tempting, but do they really make a bad deal much better? Experts say that often the answer is no.

Take a recent offer from American Airlines that offered 25% bonus miles -- which would post to the account six to eight weeks after the end of the promotion -- with the purchase of at least 20,000 miles. Even with the bonus miles factored in, each mile still costs 2 cents or more, depending on the number of miles purchased. "If you're buying their miles, you're probably depleting your account, so this is just a few thousand extra miles in your account for later," Leff says. "You're not buying the miles for the bonus. It's just a bit of a sweetener."

However, there are great offers that come along every now and then, experts say. For example, US Airways has offered 100% bonuses where the miles post instantly.

"When those happen, that can actually be a reason to just buy miles for its own sake," Leff says. "Even if you have no miles in your account, you might be in a place to buy an award ticket" -- even if you have zero points in your account -- for much less than the ticket would cost. "You get something like double the miles." Winship, who says Delta has offered a similar deal, agrees: "In that case, purchasing a significant number of miles make actually make financial sense."

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