American Airlines is running a promotion through April 30 offering 25-40% bonus miles depending on the number you purchase. The best value is 26,000 bonus miles after purchasing 65,000 — a total of 91,000 miles or almost enough for a round-trip business class ticket to many international destinations.
However, the cost is still $1,961.56 after including mandatory taxes and a processing fee, or roughly 2.16 cents per mile. And when you look at it in reverse — buying 91,000 miles and paying for only 65,000 — it becomes apparent that the discount is just 29% rather than a more impressive 40%.
I think this is too much to pay unless you have a specific need in mind. Also be aware that you are limited to purchasing 80,000 miles each year (not including bonus miles) so by taking advantage of this promotion you may have to pass on a better deal later. American Airlines has had better sales in recent months, and US Airways is apparently still selling miles at relatively good value since the merger closed. (Whether US Airways continues to do so after joining oneworld is uncertain, but for the time being they have largely preserved their award chart and I expect the sales to continue, as well.)
