PPI really depends on the viewing distance. You're usually very close to a phone but much further away from a 27" monitor and further away still from a 65" TV.

I'd say for a large 13-inch ebook reader, a 167ppi grayscale display (for anti-aliasing) is pretty good.

Until very recently all ebook readers were around 167ppi (most of them 6 inches diagonally) and most people were OK with it.

The high prices are due to the eInk panels, they must have a very low yield at large sizes making them very expensive.