There's R&D, and then there's the actual manufacturing. Sure, prices go down after a while, but you can track costs accordingly.
If you know (because you have blue prints) that the materials will cost 1,000,000 to build the plane (because your suppliers gave your price quotes), you can have a very good idea of what that plane will cost before putting it on the assembly line.
If you know (because you have blue prints) that the materials will cost 1,000,000 to build the plane (because your suppliers gave your price quotes), you can have a very good idea of what that plane will cost before putting it on the assembly line.