Well, I don't know. A decent tablet goes for $300. I could see myself paying +33% for significantly increased battery life. On mobile devices, the screen eats most of the battery charge, therefore also contributing most to the battery's degredation over years. An e-ink tablet could potentially last much longer.

It's not cheap but for me the added productivity of so many notes and being able to read long pdf documents comfortably makes it well with it.