You get up to 35,000 bonus miles when buying 75,000 miles for a total of 105,000 miles by March 3, 2015, and can get an additional 10% off by purchasing by February 9.
The price of a miles under this offer is 2.1 cents each.
It is difficult to get too excited at buying miles at 2.1 cents each, although it can make sense under some circumstances such as topping up your account or when redeeming for flight(s) between expensive city pairs.
Here are the terms and conditions of the US Airways promotion:
Dividend Miles members must purchase at least 20,000 Dividend Miles or more in a single transaction from the Buy or Gift Miles program beginning 12:00:01 am CT on February 2, 2015 to 11:00 pm CT February 9, 2015, to be eligible for bonus miles. The applicable bonus miles are awarded to the recipient for Gift Miles transactions. The maximum number of Buy and Gift Bonus miles that can be received by one member for this offer is 50,000 bonus miles. All miles purchased through the Buy Miles or Gift Miles program beyond this limit will not be eligible for a bonus. All purchases are non-refundable. Bonus miles will be awarded upon completion of the transaction. Please note there is a $30 non-refundable transaction fee on all Buy and Gift purchases associated with this promotion. All transaction inquiries must be submitted to US Airways Dividend Miles within 90 days from end of promotion. Offer is subject to change. Offer cannot be combined with any other offer. Share transactions are not eligible for bonus. Only purchases made online are eligible for the bonus. Please note that Dividend Miles accounts less than 12 days old are not permitted to Buy, Share or Gift miles. For Gift and Share transactions, the recipient account must also be 12 days old. All US Airways Dividend Miles terms and conditions apply. Visit usairways.com/dividendmilesfor details.