Igor said:

I can suggest a scheme where a fee would be attached to the message, and the recipient would be able to get the money only after having read and understood the message.

For example, you might receive the following message (it could be encrypted by your public key to preserve the cash from the MITM):

do get them to read) will have a very high response rate. That may justify adding postage to their letters.

The advantages are obvious. The disadvantage is a possibility of someone stealing the money on the way, if the letters are not encrypted.

That would be great. Sit home all day and hit web pages for a living.