Which small metal parts just cost too much?

What are the small metal components that pop up in plumbing projects and just cost too much money because of their special design or complexity?

Small fittings, couplings, adapters, nozzles, etc. which are presumably hard to manufacture. What if these could be 3D printed in metal for way less money? Stainless steel or titanium. Would love to hear about some pain points

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