I got myself a https://paperd.ink: a 4.2″ screen with battery in a printed case for $90. I wrote a small calendar application for it to replace the paper calendar at home: https://suffix.be/blog/eink-calendar
I make and sell an eink calendar (and smart display) for $149, complete with mobile apps and google calendar sync.
https://shop.invisible-computers.com/products/invisible-cale...
The display is 7.5 inch.
This is still a side project for me though I am thinking of ways to turn this into a self-sustaining business.
A battery adds a lot of complexity and cost for testing, certification and shipping. And it adds another failure mode. So I decided against it for the current version.