Most repair/service technicians charge $200-$300 to fully service a mechanical camera (my price is typically $200 plus parts, if required). Some who work on things like Rolleiflexes or Hasselblads may charge $400 or more. I’d like to say that I make a living wage doing this, but I don’t: this pays slightly more than $15 an hour. Only those who charge significantly more than I do are “making a living” doing it.
A small percentage (less than 5%) of my clients decide that it’s “not worth it” to get their camera serviced/repaired, and that’s entirely a personal choice. If a client has a personal connection to the camera (many do), then cost isn’t as much of an issue, since they plan on keeping and using the camera. But some people pick up a camera for $25 at a thrift store on a whim and they have far less interest in spending $200 or more to get it back into good condition. I don’t blame them for feeling that way - they’re not particularly invested in the device. Maybe they figured they would get it serviced and then flip it to make some money, but realize the odds of recouping the money is slim once they’ve paid to service it. I can understand not wanting to go that route either.
It will almost always be true that the cost of properly, completely servicing a camera will be close to, or greater than the value of the camera. But the vast majority of people who invest in servicing a camera are not doing it with reselling it for profit in mind.