Low sales volumes is one reason, economies of scale do not obtain.
Then the metal to electronics ratio is high and that metal has to be precision engineered so that all modules fit all power cases (and you don't want the front looking like the hind leg of a dog!).
Many 500 modules are scaled versions of famous outboard gear and some very clever design had to be used to squeeze the original functionality into the wee space AND have the thing operable by adult, male digits! Such design time/expertise has to be paid for.
But I agree that some manufacturers might be taking the proverbial! It is hard to justify £800 ($1000?) for the Focusrite Red One pre amp and the recent review in SoS said as much.....BUT! That pre contains some expensive traffs and switches. I bet if you sat down and priced every component, nut, bolt washer and screw, asked a local engineering works what they would want to mill 100 of those fronts and make the case wrap you would have a shock, and you have not yet designed and built the PCB yet!
But, you never know! Behringer could be on the case!
Dave.