If it were possible to charge for email, how much would the cost have to be to deter spammers? Can anyone think of any way to put a lower bound on it?

What I'm getting at is, I think that almost any cost would put a stop to the run-of-the-mill spammers, the people selling Viagra. I believe they have to send a million email in order to make a single sale. So the cost could be as low as 1/4 of a penny per email and that would mean it costs them $2,500 to send spam. That would put them out of business. So I wonder just how low of a charge would still work. 1/10th of a penny?

But what about the stock spammers? I linked to a study in a previous spam thread that showed that stock spammers always make about 6% profit. So for them, no charge for email would be deterrent so long as they had enough cash invested so that the 6% profit covered the cost of the email. For example, if they could invest $42,000 in the stock, then they would make $2520 after sending the spam - enough to cover the cost of a million spam messages.

Do you think that the stock spammers have that kind of money?