The cost isn’t just in the actual fiber cable here but in the connection end points and their termination, plus the device you’d be hooking it up to and the size of said device to decode whatever you needed decoded into actual sound you can hear. The optics device adds cost (which I probably should have mentioned). Repair is also more expensive which is why generally we cap and install along side instead of attempting repair. When you add in the cost of manufacturing or installing it, comparitively copper isn’t just cheaper by raw material (that cheapness depends on the scale), but also because of everything that goes into installing and using it.

This is why there is a cost trade off where you get benefits: Less Weight, Immunity to Electronic Noise, Stronger Signal, Electrical Isolation, Environmental Protection, Improved Safety, Overall System Economy, Long Term Cost Benefits.

But a device the size of a regular covert microphone would still need other components. I may have made the mistake of comparing this to like a button hole mic or something similar rather than a professional microphone (which is also expensive as hell).

Besides, how much more copper do you think you’d be using in a traditional mic than in a fiber optic equivalent?