I imagine the point is that if you make the price high enough, the ones who don't want it enough will go away, and the ones that do will want it enough that it's worth your while. You can set the price to whatever threshold is high enough that you'd find the money worthwhile.

That ignores the entire set of people who are not contacting you to get something out of you but rather to help you or for any number of social purposes. Those will be priced out much faster than dedicated spammers - most of them by any amount above $0.

>the ones who don't want it enough will go away, and the ones that do will want it enough that it's worth your while..

Counterpoint: if you think you need to pay for my attention, that's a negative signal for what you're asking me. If I'm giving 10 vendors a shot at my business, I'm not going to pay money for the right to give you that opportunity?

On the other side, the only person paying $5 to ask me something is probably someone getting a lot of no's elsewhere. That, or what I offer is so valuable that people are willing to pay for it. But that's not most people.

Bill Gates wrote about paying to send you mail in his 1995 book "The Road Ahead" (yeah this is an old old idea). His expansion was that you could refund the sender's "stamp" if you consider their email worth your while.

pretty much this. The idea is that putting work / resources behind the sent message translates into more relevant messages getting into the owner's inbox.