This is how embedded development kits work
As a former embedded hardware/software developer, I can tell you that this is how embedded development kits work, price-wise.
Tech companies like E-Ink tend to price their dev. kits quite high in order to attract "serious product developers". They aren't interested in selling to the hobbyist because the hobbyist isn't going to place an order for 10,000 display units. So, the easiest way to discourage the hobbyist segment is to price the kits high. Amazon, Sony & others can afford the pay the premium.
Also, when developing a product based on a kit like this, you usually need a fair bit of technical support from the manufacturer. We're not talking level 1 tech support, we're talking support from the actual hardware engineers themselves - so, the cost of that support is also factored into the kit.
It sucks, but that's usually how it goes...