Through August 15, US Airways is offering a 100% bonus on purchased miles.
The cost is 2.5 cents per mile plus a $30 processing fee. Purchasing 50,000 miles costs $1280 and yields 100,000 miles — a cost of $0.0128 cents per mile.
I do not recommend warehousing US Airways miles, they’re doing all they can at the moment to goose cashflow because they’re the most financially vulnerable major carrier. Assuming things improve for them, I expect future changes to their award chart given how generous they’re being at the moment — 50% bonus on transferred hotel points, double miles on shopping, and now double miles on straight purchases.
At the same time, if you’re looking to top off for an international premium class Star Alliance award, now’s a pretty good time to do that. Even starting from scratch, $1030 buys enough miles for a business class ticket from the US to Europe. And with lucrative personal and small business visa offers from Barclays Bank available, not to mention the hotel and shopping bonuses, no reason you need to start from scratch.