Bosch CX4 motors are a mere £550.

You get this with anything that hasn't been mass-produced at a large scale and commodified for a long time. It's more remarkable that something like a car (or a mobile phone, or a CPU) is cheap than that something like an electric bike motor is expensive. And it could be that this is a poor application for this type of motor technology and that a better type will be developed and that we'll look back on this as an expensive relic that was holding us back.