Most airlines give frequent flyer miles for traveling and using their credit cards. They will also sell them for cash; so should you buy? The math, at first doesn’t look promising. The average well spent frequent flyer mile is worth 1 ½ cents. Most airlines sell the miles for double that, 3 cents. You get about half what you pay for. But if you work the system you can do better. Miles are often put on sale so your cost can drop to 2 to 2 ½ cents per mile. Now if you just need a few more miles to make a frequent flyer trip -say 1,000 more miles- the slight up charge can be worth it because you can purchase a $600 dollar ticket for the miles you have on hand and another 30 bucks. Strategic mile buying works, buying just because you can, will cost you.