US Airways usually offers a bonus of some sort for shared miles or purchased miles. When they run a 100% bonus on purchased miles the cost to buy miles directly is 1.87 cents apiece, something that many consider.

Their best offer is a 100% bonus on sharing miles — transferring miles from one account to another, with the amount of miles transferred being matched by the same number of additional miles. That lets you buy additional miles at just a penny apiece.

April’s offer isn’t as good as either one, but will be strategically useful to some that need to top off an account towards an award or clear out an orphan account with just a few miles in it.

Limited time offer: Give a 50% bonus when you Share Miles!
Share miles from from April 1 to April 30, 2013 and your friend or family member will get 50% MORE in bonus miles — up to 25,000 miles.

Transferring miles costs a penny per mile. And they give you a 50% bonus. Which means if all you’re looking at is the cost to generate new miles, the price is 2 cents per mile. For instance,

• Transferring 1000 miles generates 500 bonus miles at a cost of $10
  
• Transferring 50,000 miles generates 25,000 bonus miles at a cost of $500

Then you have to add a $30 processing fee per transaction, which makes the per-mile cost higher (and much higher on small transactions since you don’t have as many miles to spread that fixed cost across).

Update: There is also 7.5% tax. Somehow I had thought that transferring miles didn’t incur tax, only purchases, but perhaps because they are adding miles to your account the transaction is treated as a purchase. So a $500 fee to transfer 50,000 miles incurs both the $30 processing fee and 7.5% tax so a total cost of $567.50, not $530 as I had originally been thinking.