Dividend Miles ordinarily sell for 3.5 cents each plus a 7.5% tax. That means if you’re purchasing between 10,000 and 19,000 miles you’re paying ~2.51 cents per mile, if you purchase between 20,000 and 29,000 miles you’re paying ~2.15 cents per mile, and if you purchase between 30,000 and 50,000 miles you’re paying ~1.88 cents per mile. As always, Dividend Miles accounts have to be at least 12 days old to take advantage of this promotion. US Airways mileage purchases are processed by points.com, so wouldn’t be categorized as airfare spend for the purposes of your credit card, for what it’s worth.
This can be a great deal if you have a specific mileage redemption in mind. 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 an amazing value. See this post for basics on redeeming Dividend Miles and this post for how to plan a 90,000 mile business class award to Asia via Europe.
For a bit of context, in January they offered a 100% bonus on purchased miles targeted exclusively to those that purchased miles in 2012, in February they offered a targeted 100% promotion as well, in March they offered a “mystery” bonus of up to a 100%, in April they offered a 50% bonus on shared miles, in May they offered a 100% bonus on purchased miles for everyone, in June they offered a 50% bonus on shared miles, and in July they offered a targeted 100% promotion.