Yes, costs of larger E-Ink display have been prohibitive; I've been told its due to the low volume production.
Btw, €100 Hisense E-ink display phones are supposedly about to hit the market[1]. Perhaps they have their own factory or has found ways to minimize the cost, if so it should be worthwhile to check them out.
[1]:https://www.youtube.com/watch?v=ZzfmvYWUwB4