RGIII: Calling All Markers

An honest question for Redskins fans: What level of success does Robert Griffin III need to achieve as a quarterback to make his acquisition worthwhile for the franchise?

For example, if he’s Elway, then he was a bargain. If he’s Achille Smith, he’s a disaster.

But what if he’s Brad Johnson? Or Joe Flacco? Or Steve McNair?

I’m looking for analogies here in terms of both numbers and longevity. And I wonder where the line is under which drafting him at the price the Redskins paid clearly becomes a mistake that, with the benefit of hindsight, would not be repeated.