The omnipresent "Sid Effect". So it's just automatic, eh? 30G every year? If Kunitz doesn't score 30G every year with Sid what makes you think a less consistent player who disappears for long stretches, even playing with talented guys (Stewart had all kinds of opportunities to play with great linemates in STL, even if not Sid), will make it happen?
Your assumption that he'd score 30G every year, is based on nothing. There is nothing in his career that suggests that will be the case.
http://sports.yahoo.com/nhl/players/3998/
I'm not saying he wouldn't do a better job than Dupuis... but he's far from a sure thing. Chris Stewart is not James Neal.
Ryan Kesler is absolutely... a sure thing. And that's why you pay the higher price.