Product designers and their assumptions influence product cost from the very start of any project. The initial stages are where the very basic decisions on the shape and design of the product are taken. The "simple and obvious" decisions such as the type of material, the production method, the wall thickness and the rough outline dimensions effectively define the overall cost of the product. Once you have decided the length, width, height, wall thickness and material type at least 80% of the product cost is already bolted into place.
Let us look at the design for the cover for a domestic appliance. It will be modelled in 3D according to what it has to contain internally and what it needs to be attractive to the customer externally. Then it will have a nominal wall thickness added — let us say 2.5mm. It could be 2.0mm, it could be 3.0mm, it doesn't really matter. What is important is that the wall thickness for such parts is often judged by history or simply that it 'looks right' for the size and shape designed.