Through November 15, 2012, American is offering 2,000 bonus miles for every 6,000 miles purchased, with a maximum of 12,000 bonus AAdvantage miles.
Through this promotion you can buy a maximum of 48,000 AAdvantage miles (36,000 base miles plus 12,000 bonus miles) at a cost of $1,099.25. That’s ~2.29 cents per mile, a rate at which I wouldn’t outright purchase AAdvantage miles since I value them at ~1.8 cents per mile.
That being said, if you’re just looking to top off an account or have a specific redemption in mind, it could make sense to take advantage of the promotion. For example, a one-way award ticket between the US and Southeast Asia on Cathay Pacific would run you 55,000 AAdvantage miles, so if you have an account with a few thousand miles already, ~$1,100 isn’t a bad deal for a one-way business class ticket.
It’s worth noting that mileage purchases with American are processed by points.com, so this wouldn’t qualify as airline spend for the purposes of a credit card.
(Tip of the hat to Gary)