What is the most expensive material used when building a rocket?

Question: What is the most expensive material used when building a rocket?
- Keywords:
  material,
  rocket
Asked by ROYLE to Norah, James, Dominic on 15 Mar 2017.
- James Harpur answered on 15 Mar 2017:
  
  The high cost of rockets apparently is mostly not due to the materials. The labour, the fact you can’t recover it and things like that are the reason it is so costly. The materials are nothing totally fancy. It’s for this reason, that Elon Musk developed the re-usable rocket which will now bring his costs down considerably. The engines also used to be the most expensive parts but he is now 3-d printing his engines.
  
  So to have a guess at your answer, the most expensive cost related to the rocket is the engineers and technicians. Actual material, might be the titanium structure due to the amount of it you need. If you’re doing it pound-for-pound, then maybe the fabricated semi-conductors.

Question: What is the most expensive material used when building a rocket?