M's have much more parts and are more complicated in construction. These two aspects ask for a more complex repair/service handling than a Barnack. That said, most M's are less old than Barnacks so age-wear would be likely to occur easier on Barnacks.
Please note that repairing camera's is very time consuming if you are doing a good job or want a good job to be done: so prices may seem steep, but are not considering the amount of work. Of course if you have a lot of old camera's, and the camera's itself are not high valued, repair/service by a third party (trained service main) will easy become too high. That's why many of us repair our own camera's. I can tell you from own broad experience: it is very rewarding.