This seems a reasonable assumption to me but I don't mean to insist I'm right.
Slow monochrome eInk panels have been around for 2 decades. Mostly built into pocket book readers, phones (like Motorola F3) and niche devices like supermarket price tags rather than computer monitors attachable with common connectors.
Okay, perhaps it's not the speed which makes them expensive, yet manufacturers and researchers mostly brag about making them faster (and more colorful) rather than making them more cheap (what I would prefer them to).
