I'm a Fitter/Machinist by trade which is why I think they are expensive. Material and hardware cost is relatively small. If I wasn't so lazy and the demand was there I would model and CNC a heap of them up.
I'd quite happily pay $250 for them... Don't get me wrong, the $800 package from UAS is decent for the average punter.
Also, bigger calipers would equate to greater stopping capacity, so that cost is not relative.