I have seen people saying that, clicks from USA and UK pay the most. Clicks from Canada also pay good. And I personally believe this concept to be true.
Think of it from a publisher's point of view. Would you want to pay the same for clicks from the developing world where their disposable income is a fraction of that in the West? It just doesn't make economic sense. G20 nations will pay better because the people living there are more likely to buy your product or service, and there are more advertisers targeting those users.
the difference between clicks in the united states and canada probably isn't that big as both would be relatively well-paying. the same goes for most clicks in other wealthy countries as well.