Cost equivalency ignores the R&D and decades of scientific progress and advancements in tooling capability. The prices of materials have shifted, but designing and manufacturing a precise propulsion system with modern CAD and simulation tools is a lot cheaper than the hand work hundreds of people used to be doing to verify much simpler engine designs. Precision machining and tools to inspect metal fatigue and imperfections have also come a long way.

Of course commercial rockets are always going to be as shoddy as they can get away with rather than as good as possible, but if it still takes SpaceX or Boeing as much money to build a rocket as it did back in the Saturn V days, they're doing something wrong.