As long as there's a way to stop self referring (which I'm not sure you can actually do) then sure this sounds good. However, simply offering a little money to refer people is not going to instant boost the game. People will naturally promote the game if they love the game. I still don't agree with two tiers it should still be one across the board flat for everyone. But it can not be required of that person to buy a spell book in order to be an affiliate.
Keep it simple and if you're going to change this it's the same affiliate % for everyone. Perhaps an approval process like submit in a form. But they also don't have to be required to buy a spell book or any game asset in order to be an affiliate but there needs to also be terms to the legitimate reason to reject an affiliate application. This goes under general rules and regulations.