It ought to be easier to get a blank slate of a small device with some compute power and a screen, like the Kindle here, without having to jailbreak something.
There is one, it's called the Raspberry Pi ecosystem, but due to the small volume and the target audience largely being not-my-own-money (think educational institutions), the price is quite detached from the production cost.
A 4.37inch E-Paper in 3 colors is $24, problem is need you to program yourself (they have code sample in python, for raspberry pi), and you need a raspberry pi, case, cables, etc.
Also, these cheap epaper displays are, of course, of lower quality (slower, lower resolution) than an kindle display.
They jump up in price pretty quickly as size goes up. The cheapest 5+ inch display I found at your link was over $40, and it's about 100PPI. It's certainly not prohibitive, but certainly priced high compared to "just jailbreak a kindle" for any remotely kindle-comparable display, right? (remotely comparable in size and resolution)
I imagine the Kindle is sold as a loss leader, plus whatever economies of scale/negotiating Amazon does pushes the price down heavily vs buying a single unit from an electronics retailer
