A good idea and sensible business. As a small non-profit we would have to opt for low price, not out of being mean but purely because we are non profit and would continue to be so even if traffic increases.
But for big sites making big money I can see it worth them paying more.
I like the way Ampxf has a lower price for non profits.
Yep this is a model that works well and is a good compromise that will get your addon into most hands. Segment based on traffic as it's a solid indicator of revenue.