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United has never been known to sell miles at a discount. In fact, normally their miles are a whopping 3.5 cents a piece plus a 7.5% tax, bringing the cost of buying 100,000 miles to $3,762.50, or nearly 3.8 cents per mile. However, they just launched a targeted sale (link only available from the email they sent out) of 15% when you buy fewer than 25,000 award miles or save 25% when you buy 25,000 award miles or more. This brings the cost of 100,000 miles down to $2,821.88 or 2.8 cents a mile, which is hardly a deal.
I can only see this making sense if you need to top up for an award or if you frequently purchase business or first class and can instead buy miles to book the awards at a much cheaper rate.
Chase sells points for 2.5 cents a piece with no tax and they can be transferred instantly to United at 1:1 ratio. Granted the only downside is that you can only buy 5,000 a month.
If you are looking to book a United flight or other Star Alliance award, buying US Airways miles at 1.9 cents with their current 50% promotion will probably make more sense. The only major downside to US Airways miles is that they don’t allow one-way awards and most of their awards have to be booked over the phone (plus their current buy miles promotion is targeted to people who purchased miles in 2011.