This idea gets floated pretty much every time the spam discussion comes up, and it would only work at the cost of the free internet. If the return from spam is

This idea gets floated pretty much every time the spam discussion comes up, and it would only work at the cost of the free internet. If the return from spam is marginally higher than the cost, there will still be spam. On the other end of the spectrum, the higher you make the cost, the more you cut out people from low income countries and poor backgrounds.