I'm pretty torn on this. Logic says it should cost more then an unlocked iPhone as the build price should be more expensive.
They may take an approach like the consoles and hope to make it back up on software purchases (app store, games, etc) but I wouldn't count on that.
However, if it's too expensive this thing won't sell except to die-hard fanatics. So, I would say they need to keep it in the $750 or below price range.