That's the root of the problem... Who would collect the fees?

Internet service providers pay for their connection to the main backbones and for their namespace registrations, but the traffic they place onto those backbones is not metered by "type" (i.e., email vs surfing).

So although a given ISP might choose to charge its customers your suggested scale, spammers would simply move to a spam-friendly ISP that did not charge per email.