While they had a targeted offer released earlier in the month, it looks like US Airways made their 100% bonus buy miles promotion available to everyone for the month of September. US Airways usually sells miles for 3.5 cents each, so through this promotion you’re paying 1.75 cents per mile, plus a 7.5% federal excise tax, for a total of ~1.88 cents per mile. You can purchase a maximum of 50,000 bonus miles through this promotion (for a total of 100,000 miles), and your Dividend Miles account has to be at least 12 days old in order to participate.
There definitely are some cases where it can make sense to buy miles from “scratch” in order to book an award ticket. For example, a business class award ticket from the US to North Asia costs just 90,000 miles, and you can route via Europe and have a stopover on the way if you’d like. That comes out to about $1,700 plus tax for a ticket to Asia in business class, which is an amazing deal.
(Tip of the hat to Gary)