Make has a great article that goes over what things to consider when wanting to manufacturer your hardware product. They bring up that the case often one of the largest costs of the product due to needing to make molds. There is the area between low and high volume manufacturing where using an off the shelf case would be cheapest but at some point getting a custom case manufactured for your product would make more sense.
“When bringing your product to market, scaling costs will likely be one of your biggest obstacles. Failing to plan for these costs will make it impossible for your product to succeed. For most products, the cost to transition from making a few prototypes to manufacturing thousands or millions of units is the most expensive obstacle.”