From now until December 31st, AA is offering bonus miles on mile purchases. Additionally, the normal 40k/year cap has been lifted to 60k miles. Miles cost $29.5625/1k (inclusive of taxes) for purchases of 6k miles or more, plus a $35 processing fee on the entire transaction.
The bonus system is tiered.
As to best amortize the $35 transaction fee, purchasing 60k miles for $1808.75 yields 90k miles, yielding a cost of 2.01cpm.