And who decides how much that should be? How do you price out the spammers (who have access to stolen funds) without pricing out the hobbyist or the NGO in a low income country?
If you want to use economics to block spam, then require that any newly registered (or newly email-sending) domain put up a cryptocurrency bond before it is allowed to send email. (All existing domains would be grandfathered in, meaning they would have an incentive to adopt this system).
If a domain is found to send too much (DKIM-signed) spam, the bond can be forfeited (slashed or donated to charity), triggered by N-of-M validators agreeing. The validators could be some mix of the biggest email providers, and non-profits like Mozilla and the ISRG. They would also be able to adjust the bond price according to how much spam they still see.