Seems that US Airways has started to sent out these targeted buy/gift offers for bonus miles instead of running global promotions every month on their website.
If you got email similar to one above (using old screenshot), you will get 100% bonus (up to 50K miles) for buying/gifting US Airways miles by September 17, 2013.
This offer is likely targeted to those that have purchased miles during US Airways previous sales. The price of a mile under this 100%bonus offer is 1.88 cents each.
Does it make sense to buy Dividend Miles at 1.88 cents each?
US Airways has number of promotions yearly for buying, gifting, and transferring in points from hotel programs to Dividend Miles.
This 1.88 cents per mile offer has been one of the lowest purchase offers. There was an offer for transferring miles in 2012 during which you could “purchase” miles at 1.1 cents each, but that offer was maxed at 50K miles.
Conclusion
If you have firm plans to use these miles and the availability exists on Star Alliance carriers, I would go ahead and buy.