E.g. $0.001 per email, paid to the recipient
Insignificant at personal scale, but a deterrent to sending low-value emails at mass scale, and double-painful when an unbalanced flow (i.e. a spammer who receives no organic email coming in)
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.
The key insight here was making it expensive for spammers, but cheap for everyone else.
Consequently, where X < Y.