How do you implement that? Where does the money go?

If you have it go to an authority, you're relying on a central authority to handle e-mail. It'll be abused faster than you can say "ICANN".

If you have it go to the receiver, then you need a way of getting the payment to them. That requires a central authority (same problem as above), or having everyone accept credit card payments (impractical with the current credit card processing framework), or a cryptographic cash mechanism. But once you bring cryptography into play, you can solve the spam problem via cryptographic signatures, which makes the whole payment system moot.