US Air last offered this promotion a couple months ago in Oct 2013. I took advantage of it then and now they are offering it again for a couple days only.
HERE IS THE LINK
Take a look at my previous post Business Class US to Asia for $567 on US Airways & Star Alliance (worth $7000+) which explains how to take advantage of this. In short, for $567.50, you can buy 50,000 points. In most cases in the past, these types of offers havent bee worth much but this one is very good, especially for those of you who where an extra 50,000 miles will get you to a business class flight, or for those of you who just want to buy miles on the cheap.