I would argue that e-readers actually do need reasonably low latencies. The reason I still read books is because I can flip through pages quickly and glance over 20 pages a second as I look for the page with a certain pattern of text or an image.
Not sure how specific you're trying to be - here's a specific example of 4.2" at $35. I actually would be interested to see a graph of cost-per-square-inch to see where the arms and legs actually do come into play.