Lots of people have proposed the idea that spam is simply an economic problem: If people have to pay to send each spam, they would send less. This is not a correct idea. I say this so flatly because people can already ask strangers to send them email with a monetary attachment. When they get email from a stranger, they autorespond to it with a message that says "Hi. Thanks for sending me email. I've never gotten email from you before, so I want you to pay me $1.17 to read your email. All you have to do is sign up for paypal, and send me $1.17 via paypal. When I get it, I'll know which email to read." The $1.17 is a key that points them to the specific email that was sent.
Nobody does this. Why? Because it doesn't work. Do you think I'm wrong? No problem! Just send $1.17 to my paypal address and include your message in Paypal's comment field. If you have too much to say, end with "to be continued...", and send the continuation to my main address.