US Airways introduced a 100 percent bonus miles offer on purchased miles earlier this year and the airline is offering the same bonus again–only this time, mileage transfers will also be doubled. Through Dec. 31, 2009, US Airways will double your mileage purchases and transfers of up to 50,000 miles.

Bottom line: In most cases, it’s cheaper to buy a flight directly, but there are some situations where buying miles makes sense. For example, US Airways has an off-peak award sale from the U.S. to Europe where a coach flight is only 35,000 miles for travel between Jan. 15 and Feb. 28, 2010. Miles cost 2.5 cents per mile plus a $30 processing fee per transaction, so you can purchase 18,000 miles and receive 36,000 miles for a total of $480–not bad for an off-peak roundtrip award ticket to any US Airways destination in Europe. If you’d rather travel roundtrip in business class to Europe by buying the miles, the cost would be $1,030 for the 80,000 miles you’d need for a Mileage Saver business class award.

The deal becomes even more interesting if you transfer miles and want a fast track to that European award. For mileage transfers, the cost is $0.01 per mile plus the $30 processing fee. If you and a fellow Dividend Miles member have 24,000 miles each in your account, you can each transfer 12,000 miles into the other’s account for $150 and you’ll both have 36,000 miles–more than enough for that discounted trip to Europe. The total cost per mile for the 12,000 miles you’ll each gain into your accounts would be $0.0125 (this includes the $30 fee for the transfer). Take note that you must have been a member of US Airways Dividend Miles for at least 10 days before you start buying miles.