Aamco was cheaper then Castrol the last I saw at Autozone. That and if Castrol buys its oils from Exxon/Mobil it is probable close if not almost the same.
I read a couple of years ago that the Motorcraft Mercon V was the only ATF that fully met the spec. I get mine at the Ford dealer for about $3.60/qt. Be sure to ask for a discount, though.