A friend tried something like this a few years ago. He built a resume site where recruiters had to pay to contract you about a job. If you didn't respond in 24 hours they got a full refund. If you responded (even if the answer was, not interested)then you and the site shared the revenue (70/30 IIRC). Each person set their own price.

I thought it was a cool idea but he launched in 2008 just as the economy tanked and he never got any traction.

Autorespond to recruiters with a link to your "updated resume", but make it an Amazon affiliate link instead. Any purchase they make in Amazon in that same day = commission

[ insert Bill Gates $1 to send emails joke ]

I believe people have talked about something similar, namely requiring cryptographic operations which are relatively cheap (time, electricity cost) to do for one email at a time, but which would make it infeasible for bulk mailing spam.