Through July 31, 2011, American is offering a 25% bonus on a minimum of 20,000 purchased AAdvantage miles (the maximum per calendar year is 40,000 miles). While the base miles purchased should post fairly quickly, it will take 6-8 weeks for the bonus points to post. American charges 2.75 cents per mile, plus a $30 transaction fee and taxes. So in this case, 50,000 miles (with the bonus) would cost you $1,105, or about 2.2 cents per mile.
That’s by no means a great deal, though it’s worth keeping in mind that American counts all miles towards million miler status, and there are rumors of that policy changing soon. So if you want to lock in million miler (or two million miler), this might be worth taking advantage of.