They don't cost the same to make though. They can't go to the supplier and say, "I want 15,000 exposed roofs, and 500 lower quality ones." Well, they could, but then the roofs would cost even more because of the ones that don't make the cut. They can, however, order 15,000 paint quality roofs. The exposed ones are of a higher quality. The ones that don't make that cut get purchased to paint. The exposed one is the more difficult to create, and so gets the higher charge. Sure there is some arbitrariness to the cost premium they place on it, but it is not the same cost to manufacture a part of sufficient quality for clearcoat only as it is to get one good enough for painting.
If you want a computer analogy, higher cost chips are manufactured on the same wafer as lower cost options that share the same die. We don't try to make the argument that the higher cost ones are the same cost to create as the lower cost ones, because we can't just make the highest quality parts (until a process becomes extremely mature, then more and more are higher quality, don't have cores that don't work, etc)
Also, I used roofs here because it was easier to type than aero bits over and over. It's the same thing though, just a different part.