Metal is expensive for a lot of reasons. If you want to do some interesting industrial design, it's more expensive to do with metal. If your idea of ID is a folded metal chasis with a stamped metal faceplate, then you could probably pull that off for about the same price as having a case injection molded, but then you have weight to deal with. That extra weight also costs money. The difference between a plastic case and a metal case might be small initially - but that multiplies quickly when you blow the volume up to 10K units.
Also, there are other issues to consider when choosing materials to make a product. Thermal conductivity, EMR, how labor intensive it is to use, how much waste is produced, etc. Metal is not always the best material to make a case out of.
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