Take a look at Twilio. NB-IoT plans at $2/month per SIM. Super easy way to get started. You can beat carriers up over pricing once you have volume. I have seen coverage as low as 50 cents per device per month.
Great info! I’d not heard of eUICC, but looks super useful. Some of the comments here suggest Ting once had an IoT plan, but https://iot.ting.com now just redirects to ting.com .