I think people have unreasonable expectations. Get a designer to produce drawings for something with a fractuion of the complexity of a model locomotive under an ISO9000 accredited process with a detailed change log and review process. You may get what you want, but at at least £300 a day expect to pay far more than £67 a shot for a set of drawings. In my job I've paid five-figure sums for the design of paths and boardwalks a lot simpler than a loco. Yet even these have to be reviewed and modified by the contractor.
Model engineers don't make designs to become rich. Most do it because, fo some, there is great satisfaction in seeing your ideas realised (and enhanced) by others. Others just see it as a challenge. A very few make a living, but not a fortune. I've spoken to a major supplier and even a prolific designer can only expect to supplement a pension.
In short, ME drawings should be seen as a service and a guide, not full production spec.
It isn't unreasonable to expect the custodians of the drawings to keep a list of notified errata, but to go beyond that is unreasonable, as only a full build exactly to drawings by a professional could get near guaranteeing accuracy.
My tuppenceworth…
Neil