In the current line up, the Sugino 'Mighty' crankarms are supposed to be forged from 2000 series aluminum. (which one not specified) The price reflects that. So along with forging vs casting, there's a matter of how fancy an alloy you get.

I'd venture a guess that the old Sugino Mighty's from the 70s and 80s were 6061 or similar, which is fairly conventional.