On American’s buy miles page it’s still listing a one-third bonus from last month:

Between now and December 31, 2011, when you buy the AAdvantage® miles you need to book your travel now, you’ll earn a special bonus — for every 6,000 miles you purchase, you’ll earn 2,000 bonus miles, up to 20,000 bonus miles!

But when you click through the January offer is available.

A 40,000 mile purchase for $1100 yields 55,000 miles — a purchase price of 2 cents a mile.

Now, I’m not stocking up at 2 cents apiece. But it’s not an unreasonable rate for topping off. Lower mileage purchase amounts are more costly per mile and less attractive.

Still, given how readily available Cathay Pacific and Etihad first class awards are much of the time, it’s not unreasonable to put an award on hold (American will hold an award without miles in your account) and then topping off the miles you need for the award — an award can generally be held for 5 days and mileage purchase can take 3 days although I believe usually takes a little bit less time than that. (For that matter, I found a couuple of Qantas A380 first class award seats yesterday… for which I’d gladly pay 2 cents a mile.)

You can only purchase 40,000 miles per account (plus in this case, 15000 bonus miles) per year, though in December they temporarily increased the cap to 60,000.

At my current AAdvantage mileage balance I’m not a buyer, but some will find this useful.