personally, you’re not comparing things of parity.
Discovery boards are as you expect, offered by STM to promote their chips. They are examples of what you can do with the processor, they are a reference architecture in themselves, but they are not a guaranteed-supply, robust commercial “module” that you can continue to rely on as a core part of a commercial product. They are a development board more than a product.
I think anyone would struggle to assemble a discovery board equivalent product for the price - they are likely loss-leading from ST, as a way to promote the platform. As I said earlier, great for a one-off or testing the waters, but this price delta means they’re not something you can then go make yourself cheaper.
If you contrast that to say an Arduino, then it’s relatively easy to go and “clone” an Arduino into your own design and expand on it, and it will likely cost you the same or less. Same with a Cerb40 - the BOM cost is similar to the cost of the device itself.
Now you’ve said this is a commercial product. I am guessing your “device” is expected to be cheap, and because we’re having this “cost” discussion that the software cost (firmware) is not very complex nor hard to write - because that is where C# will help you decrease the overall cost of the solution, allowing you to have potentially more “cross purpose” developers produce code quicker and to leverage the benefits the Visual Studio development and debugging environment give you. Perhaps the BOM cost isn’t the most important thing in your cost base after all - I don’t know.