Unfortunately that is insignificant at the larger end too.

An accountant would just look at that, figure out the click-through rate and plug it in to weigh it up against the CPM/CTR of equivalent advertising.

And you'd lose any "ethical" arguments against spam. You'd unlock a tidal wave of companies who would now feel justified in spamming because they're paying to do so.

Just as companies don't feel ashamed to bleed adverts into every other waking space.

And, as we all know, charging money for a blue checkmark totally solved the bot problem on Twitter.

You don't need to re-pay for the blue checkmark for everyone who reads your post.

The key insight here was making it expensive for spammers, but cheap for everyone else.

The point I'm making is that is just a cost, so X is the money made from spam, and Y is how much it costs to send it, if X > Y, you're still getting spam. Companies pay MailChimp and every one in that whole ecosystem money. adding another cost is just adding another mouth to feed.

Yes. And the worst spam all meets the criteria of massive distributions of low-value email.

Consequently, where X < Y.

...which changes the economics of sending the spam email. Surely some of them will be "valuable" enough to send even with the added cost; however, a measure doesn't need to be 100% effective to be useful.