Based on my meager experience in this area (mostly having been involved in the mechanical keyboard enthusiast scene where there's a lot of small-batch [e.g. 50-

Based on my meager experience in this area (mostly having been involved in the mechanical keyboard enthusiast scene where there's a lot of small-batch [e.g. 50-5,000 units] manufacturing), the killer for CNC machining is the complexity of the shape being machined and the tolerances desired. A simple flat rounded rect is relatively easy/cheap, but if you go for a more 3D sort of shape with complex curves cost will go through the roof. It's similar with tolerances, with large margins being cheap and low being high.