I'm not an expert, but I'd imagine a lot of it was R&D work, but also much of it was precision manufacturing that made it expensive. I do like the idea of making a bunch of identical telescopes to drive down cost and allow for more astronomers to have time on the best telescopes. We wouldn't need to make another JWST though, because Starship could just fit an unfolded telescope of that size in its fairing. The LUVOIR-A telescope is sort of a scaled-up JWST, with the folded mirrors, but from what I understand it is different enough that there are many new engineering challenges.