Through December 20, 2013 (11:59PM PT, specifically), US Airways is offering up to a 100% bonus on the purchase of Dividend Miles. The bonuses are tiered, as follows:
Buy 10,000 – 19,000 Dividend Miles, get a 50% Bonus
Buy 20,000 – 29,000 Dividend Miles, get a 75% Bonus
Buy 30,000 – 50,000 Dividend Miles, get a 100% Bonus
So in order to get the 100% bonus you have to purchase at least 30,000 miles. US Airways ordinarily charges 3.5 cents per purchased mile, plus a 7.5% tax, for a total of ~3.76 cents per mile. So with a 100% bonus you’re paying ~1.88 cents per mile, which is fairly good. The deal gets less good if you just purchase enough miles for the 50% (~2.82 cents per mile) or 75% bonus (~2.35 cents per mile).
Earlier in the month US Airways ran a 100% bonus on shared miles, which was a much better deal since it was basically an opportunity to pick up miles for ~1.1 cents each. The “catch” is that you needed an existing mileage balance in order to share miles, so if you didn’t have miles in any US Airways account that wasn’t especially useful. We’ve seen two such 100% share miles promotions in the past several months, which is more than normal.
Anyway, 1.88 cents per mile isn’t a rate at which I’d speculatively buy miles, but if you have a specific use in mind it can still be a very good deal. US Airways charges just 90,000 miles for business class between the US and North Asia, and you can route via Europe with a stopover there. It’s anyone’s guess how much longer till they devalue their award chart, but spending under $1,900 to get enough miles for such a ticket is a bargain, assuming you can lock it in soon.
As a reminder, US Airways mileage purchases are processed by points.com, so wouldn’t qualify as airfare spend for the purposes of your credit card (so if you purchase them you’re best off using a card that’s rewarding for “everyday” spend).