US Airways is almost always offering a 100% bonus on purchased miles in one form or another, bringing down the cost of acquired miles to ~1.88 cents each. However, the single most lucrative promotion they seem to offer about once a year is a 100% bonus on shared miles, which they just announced through October 15, 2013.
The promotion is pretty straightforward and not targeted, fortunately. They’ll give you a 100% bonus on all shared miles, up to a total of 50,000. The cost to share miles is one cent per mile, plus a 7.5% tax, plus a $30 transaction fee.
So what this means is that if you transfer 50,000 miles from one account to another you’d pay a total of $567.50, and that account would then have 100,000 miles. So basically you “generated” 50,000 miles at a cost of $567.50, which is just ~1.13 cents per mile.
Obviously this promotion is only useful if you have US Airways miles in your account, but it could very well make sense to transfer points from Starwood, for example, in order to be able to take advantage of this promotion. It seems you can make as many transfers as you’d like under this promotion, and that the only major restriction is that members can only receive 50,000 miles per account — there are no limits as to how many miles can be transferred out of an account. The other restriction, which is standard for US Airways promotions, is that your account needs to be open for at least 12 days before they’ll let you take part in a promotion, so if you don’t have a US Airways account yet, it would make sense to sign up ASAP.
To put this into perspective, US Airways charges just 90,000 miles for business class from the US to North Asia, and you can route via Europe with a stopover there. So if you can generate miles at 1.13 cents each, that’s the equivalent of paying ~$1,000 for such a ticket. For tips on booking such a ticket, see this post.
This is absolutely as good as US Airways promotions to purchase miles get, and a rate at which I’d speculatively purchase miles even without an immediate use.
(Tip of the hat to Gary)