I suspect the cost comes from manufacturering tolerances, miniaturisation, and Quality of components.
If you simply want to measure the strain on a lever and don’t care about size, weight, form etc then it could be relatively cheap. But to measure the rotational force would need 2 or 3 gauges, that have to suffer varying conditions and a huge amount of vibration and still give accurate measurements. Oh and they need to be small enough to fit in a pedal, crank or hub, and cyclists don’t like weight so they need to be light.
Things will get cheaper I guess, but I can’t imagine there is much need outside of power meters for micro strain gauges