Stop spam, charge to send emails?

Just a thought, reckon it would stop spam almost overnight, if ISPs/email hosts charge per x amount of email sent.
Genuine users who send a few/day aren’t going to be charged much, ( I only average about 20 outside my LAN/day, I know there will be lots of people legitimately sending 5, 10 or 20 times that )but the junk senders plundering our mailserver/bandwidth/pc/time resources will be. If it starts costing $$ to send 10000 emails, it will be time to stop and think.
It would probably kill the free email services, but then thats where half the junk in my mailbox replies to anyway. I for one would be happy to have my internet access plans renegotiated to include numbers of emails sent.