It isn't spam if they're paying for the traffic. Commercial advertisement through electronic mail is only evil because it is forced on someone against their wishes and on their dime. The current situation is much like a telemarketer calling long distance collect with billing done automatically, and you can't hang up until they're done with their spiel.
If it were standard practice for email recipients to charge the sender an ecash fee (waived if they thought the mail worth their time), it would make things much more interesting.
a very interesting proposal (I believe Bill Gates may have even suggested this in his book), but keep in mind you seem to be mixing different ideas here. there is a cost involved in delivering a message associated with pure communications costs. then there may be a cost imposed on someone to obtain the attention. the latter is arbitrary. the former is pretty well established based on internet infrastructure. what you might keep in mind is the following: in a public forum, where "spam" was originally invented, who decides how much it costs to post? now lets say we could quantify the communications costs. ok fine, this comes out to $.05/msg (a large example number). that is still economically viable for someone to "spam". to a mail marketer this would be a really great deal. hence a system that only charges communications fees is unlikely to prevent spam, imho. furthermore, in a public forum, you aren't very easily going to be able to implement "arbitrary" charges like I wrote about. so charging for email to one's mailbox is one possible way to deal with spam, but it's hard to see how one could apply this to public forums. and in many ways, the spam problem is most difficult to solve there. but I like the thinking on methods of trying to solve the spam problem. I do believe it is solvable. it's just that its such an insanely difficult problem to solve. it's a good example of a problem that gets worse when the span of the software grows larger.