"A penny an email" is a figure that's tossed out, but it could be far lower. And let's ignore the technical aspects of how you'd go about collecting it (SMTP really isn't designed to make such payments easy, though we could bolt, staple, glue, and wire on yet another patch to the mess).
The point is that abusive email -- brain-dead bulk spam, and even annoying recruiter pitches and "real" business adverts -- are sent in far higher volume than even the all but the very largest of mailing lists. The standard figure bandied about for most of the past decade is that some high-90% of all email is spam. Let's say 97% (as Microsoft reports: http://news.bbc.co.uk/2/hi/technology/7988579.stm).
Spammers are generating returns based on a small fraction of a percent response rate on an overwhelming volume of mail, which is close to free. Sender costs are estimated at $0.00001 (http://www.clickz.com/clickz/column/2138759/make-spammers-pa...). That's 1/1000th of a peny per mail, or a cent per mil (thousand mails).
I suspect that raising that cost by even just a few cents per mil would be sufficient to put the brakes on most spam. For personal email, that would literally be a few pennies a year for even a high-volume correspondent. Protocols for mailing lists and other legitimate noncommercial use would probably rely on offsetting costs to users.
In reality, proposals such as hash-cash, greymilter, targrubing, and the like extract computational work from an untrusted SMTP sender as a requirement for accepting mail. It's a pretty effective way to put the brakes on high-volume delivery. Computational power is a pretty reasonable proxy for coin of the realm.
And if that's not enough ... there's plenty that's going on now with DKIM and other header validation that would serve to identify a sender. Extend that such that you don't accept non-signed mail (except at a very, very, very slow rate) and that you can associate a designated signer with a verified escrow account, and you've got much of the nuts and bolts of putting a system into place. Not that I see this as entirely desirable, but we may have to go there.