For some research funding is mostly the salaries of the people doing the research.

What research works that way?

large parts of Computer science, mathematics, theoretical physics, economics, some humanities research and I'm probably missing a ton.

So applied math, math, applied math, applied math, and maybe some others.

That makes sense, although I'd argue that at least in the realm of HEP theoretical physics has extraordinarily expensive kit compared to what scientists make. See: The LHC.

Reducing all these fields to mathematics is incredibly reductive. I don't think it's in good faith.

I think it's telling that "Researchers cost more than any other aspect of research" seems to involve primarily math as inputs and outputs. Like I said the obvious outlier is theoretical physics because that tends to involve significant resources both to explore (supercomputers) and verify (colliders, telescopes, interferometers, etc). Most people involved in research in any number of fields do not find that their collective salaries are the majority of the cost required; math is an outlier in many cases.