Isn't this the opposite of what I want? I don't want people willing to pay getting into my inbox. Those are the people who think they can get more from me somehow. Those are exactly the people I want to not be in my inbox.
You will get fewer of them because they all must pay a cost for each message sent. Spam works now because it's free.
I imagine the point is that if you make the price high enough, the ones who don't want it enough will go away, and the ones that do will want it enough that it's worth your while. You can set the price to whatever threshold is high enough that you'd find the money worthwhile.