~$250 for the board + $40 for Soarer's converter 
A Teensy is only about $22 shipped and the Chinese knock-offs are as little as $5.
For $40 you can get one of Orihalcon's cable converters, but you don't need any of that, AT speaks the right language already.
A straight AT-to-PS/2 adapter is all you need and even a PS/2-to-USB can piggyback off of that.