Through January 31, 2013, US Airways is offering a 100% bonus on the purchase of Dividend Miles to those that purchased miles with them in 2012.
Through this promotion miles can be purchased for ~1.88 cents each including tax. You can purchase a maximum of 100,000 miles through this promotion (50,000 base miles and then 50,000 bonus miles).
This can be a great deal if you have a specific mileage redemption in mind, and can be an even better deal if you’re just topping off a mileage account. Flying from the US to North Asia via Europe costs just 90,000 miles in business class, so if you outright buy those miles through this promotion you’ll pay $1,700, which is really good. See this post for basics on redeeming Dividend Miles.