Disclosure: I worked with Visionect a few years ago to develop an e-ink restaurant menu.
The latest 6" Kindle with WiFi is ~$80 (on Prime Day it was going for ~$50). Given that it has WiFi and there are some Jailbreaks for Kindle you could get a Kindle to do this same thing. If you want better contrast you could upgrade to the Kindle Paperwhite for $120. That's anywhere from 8% to 20% of the cost of the Visionect system.
EDIT: Turns out the Kindle can do quite a bit including running a web server, web browser, SSH and much more so this should be easily doable.
[1]: http://www.mobileread.com/forums/showthread.php?t=128704
The Visionect kit is cheaper, more capable and better supported than comparable dev kits.
[0] https://www.mainstreet.com/article/exclusive-amazon-s-79-kin...
How long do you think a kindle would last out in the elements? What is the battery like? What is the I/O like? Security (physical and software)? Is the screen contrast similar? SDK support? Customizability? 60601 ? All of these things are potentially expensive to provide.
Anyway, it may not be the right dev kit for your applications, but the fact that they share a display technology is a tiny piece of the puzzle. As you note, for you, the kindle might do a lot more for less cost (in which case, why not just use one?)
In my experience, looking at mass produced consumer electronics is a pretty terrible way to spitball the cost of anything that isn't mass produced consumer electronics.
It's very common for these companies to sell dev kits at high cost because the volume is tiny and the overhead of filling the order for them is large. This is nothing new.
I feel that a lot restaurants that change menus frequently pride themselves on printing new menus every day. When you get the menu, it seems to reinforce the idea that everything - the menu, the ingredients, etc. - is fresh. Not sure that holding an e-ink reader that's been handled thousands of times by others would give the same feeling, but maybe I'm wrong?
The company seems to have pivoted to a more lucrative business model though (Slovenian population is 2MM, with GDP per capita somewhere around 17000 EUR).