I'm pretty sure it's just in-demand. It wasn't cheap to begin with, and the number of fans that want one is apparently pretty high.
Came with a little trampy girl and was boxed in robot mode standing up. THey were very rare because they were not quite Binaltech and not quite KISS. It was like a lazy "yeah we know we should release these repaints." by Takara. We never really got Alternator Sunstreaker.
Fine, fine, fine I bought it. But seriously thanks for all the quick replies. Well my Alt/BT collection is now complete (minus the red Jazz, and broadblast but I didnt want them anyways). I think I spent more money on these last 6 (Rodimus, Mirage, Swerve, Autorooper, Alert, and Nemesis Prime) then my entire collection!