It gets more cost-effective if combined with [semi-]automated tooling for verification, testing, and refactoring. Functional style makes these things easier. That's the draw in for me. So, I collect info on FP and verification followed by code generators and equivalence provers/testing for imperative and low-level forms of it. Examples include seL4 using Haskell->C, Lammich's functional->imperative converter for data structures, or maybe even compilers like CakeML.