Through October 10, 2013, American is offering variable bonuses on the sharing of AAdvantage miles.
The percent bonus varies based on the number of miles you share, as follows:
Not only does the bonus vary based on how many miles you share, but the cost per transferred mile varies based on how many miles you share.
So for example if you transfer 50,000 miles that will cost $580 (including the $30 processing fee), and you’d pick up 15,000 bonus miles. If you were looking at this exclusively from the perspective of outright buying miles that’s a horrible value, since you’re paying almost four cents per “acquired” mile.
This isn’t a promotion I’d take advantage of as a way to acquire AAdvantage miles, but if you have to share AAdvantage miles anyway, it’s certainly worth keeping this in mind.