US Airways is running tiered bonuses for mileage purchases through May 31.

• Buy/gift 5,000 – 9,999 miles, get a 25% bonus
  
• Buy/gift 10,000 – 19,999 miles, get a 50% bonus
  
• Buy/gift 20,000 – 29,999 miles, get a 75% bonus
  
• Buy/gift 30,000 – 50,000 miles, get a 100% bonus

For purchases of 50,000 miles you get 50,000 bonus miles, and your 100,000 miles cost you 1.88 cents per mile.

(Incidentally, through May 17 American will sell you miles at 2 cents apiece.)

US Airways has been dubbed the ‘consolidator of Star Alliance premium class seats’ since seats that aren’t expected to be sold (distressed inventory) tend to be offered as awards, and US Airways has been the most aggressive about letting members straight up buy those seats cheaply via purchased miles — even putting awards on hold before buying the miles (which post instantly).

Although they’re no longer the only one in the game. When AviancaTaca sells miles with a 100% bonus the cost is 1.5 cents per mile, and they allow one-way awards without Starnet blocking though if you can’t book the flights you want online (and you can’t book awards in a mixed class of service at all) their call centers can be difficult to worth with.

US Airways mileage purchases are processed by points.com which means they don’t show up as airfare, and thus don’t earn bonuses from credit cards like Chase Sapphire Preferred (double points) or American Express Premier Rewards Gold (triple points) that bonus airfare spend.

Buying miles at a 100% bonus from US Airways used to be an amazing deal. Three years ago their award chart was less expensive (e.g. 80,000 miles for business class to Europe). And prior to September 2010 miles normally cost 2.5 cents apiece.

In October 2011 they raised the standard price of miles to 3.5 cents apiece.

Since US Airways runs mileage purchase bonuses nearly every month, though sometimes targeted and other times in varying amounts, they’ve more or less jacked up the price only to be able to discount it, making the discounts look better than they actually are. Make no mistake, they can still be a good deal, but not good enough in my view to stock up miles speculatively. Instead, I’d use this deal to top off an account towards a specific award or to buy outright a ticket that you can immediately book (getting airfare at a discount).

Here’s my guide to using US Airways miles to book awards.

(HT: Lucky)