Let's Encrypt generates ~7M certs/day[0], a cert is only good for 90 days (~a quarter) so let's use 90 days as a window.

  7M/day * 90 = 620M/quarter

So that's ~2.5B certs issued per year (knowing that these are often reissues, but you get the notification each time you approach the expiry).

Assuming only one message (not what happens, you get more than one notice.. especially if you let it expire, you get at least two follow up messages) per cert:

  2.5B * $.1/1000 = $250K/yr

Some users don't provide email addresses, some don't provide valid ones (doesn't mean their infra doesn't have to try to contact, at least for the first expiry), some use a renewal script that renews before the email (9 days prior to expiry I think?), some don't care to renew (I'm sure LE is used in throw away cases where they don't care to renew, but the 3+ emails were still dutifully sent).

But for 1 email per issued cert it's > $20k/month to send these messages with SES pricing.

There's also the TLS validity halving (well.. 90 days -> 47 days) looming, which in some way helps with the revocation servers, but would also double their (former) email costs. And then there's future proposals that would half or ever quarter that lifetime again (once again multiplying their email costs). At some point LE would just be an Amazon SES support system (like DVD-Netflix was for postal services).

[0]: https://letsencrypt.org/stats/