Who’s got the cheapest miles? The answer, of course, depends on what you’re looking for. Right now, five of the Big Six airlines (wherefore art thou, Continental?) allow program members to purchase miles to “top off” their accounts and reach award levels faster. How many miles you can buy and what you pay for those miles, however, varies drastically.

Nearly all the programs allow members to purchase miles in 1,000-mile increments, with a minimum of 1,000 and a maximum of 15,000. The exceptions are Northwest WorldPerks (increments of 2,500 miles and a maximum of 10,000) and US Airways Dividend Miles (2,000-mile minimum).

Of course, Uncle Sam wants in on the action, so expect a 7.5 percent federal excise tax on your purchase.

And each program charges a transaction fee, usually $25, no matter the size of your purchase.

This is where it starts to get tricky. If you’re buying 15,000 miles, an extra $25 is much less painful than if you’re only buying 1,000 miles. Needless to say, the total cost per mile will vary drastically depending on the size of your purchase.

For example, with American AAdvantage, a 1,000-mile purchase ($27.50 plus $2.06 in taxes plus a $25 transaction fee) will total about 5.46 cents per mile. Jump up to a 15,000-mile purchase though ($375 plus $28.13 in taxes plus the $25 fee), and you’re only looking at about 2.85 cents per mile.

The notable, and laudable, exception is United Mileage Plus, which charges a uniform transaction fee of .1 cents per mile, which keeps your cost per mile at a uniform 2.79 cents.

Not all of us are bookkeepers, or want to be. If math gives you hives, here are the results in black and white: American AAdvantage miles run from 5.46 to 2.85 cents each; Delta SkyMiles run 5.46 to 3.13 cents each; Northwest WorldPerks miles, from 3.8 to 3.05 cents; United Mileage Plus, a flat 2.79 cents; and US Airways Dividend Miles, 4.25 to 3.17 cents.

Perhaps not surprisingly, the smaller airlines offer some pretty good deals of their own. America West FlightFund, for example, only charges a $15 transaction fee and allows you to purchase up to 15,000 miles for 4.19 cents to 2.79 cents per mile. Alaska Airlines Mileage Plan offers perhaps the best deal, waiving the $25 transaction fee entirely if you purchase online. With Mileage Plan, you can purchase up to 10,000 miles for a uniform 2.69 cents per mile — the best price among all programs.

Most programs offer mileage purchase options on an ongoing basis, but be warned: WorldPerks’ offer ends August 31, and Mileage Plus, the best deal of the big airlines, stops selling its miles on Dec. 31.