An AAdvantage member can purchase up to 40,000 miles per calendar year.
They cost from USD0.025 per mile.
(10,000 will set one back USD250)
An SPG member can purchase up to 20,000 SPG points for USD0.035 each. With the 5k bonus for transferring 20K or more points to AAdvantage, those 20,000 can be converted to 25,000 AAdvantage points. This is a effective rate of USD0.028 per Addvantage mile.
Unless you are after more than 40,000 AAdvantage miles, purchasing directly into ones AA account is a less expensive option.