> What if they took the JWST design ($10 billion in development costs!) and made 10 or 100 more copies? How much would each additional copy cost to build and launch?

There's presumably a way to estimate that. But you'd also want to ask if it's even useful to have 10 or 100 copies of JWST? Even if the unit cost is lower, the total program cost would still be greater than the $10b for one. The astronomical community is probably not large enough to fully utilize 10 JWSTs, let alone 100. I'm sure we could come up with enough observations to keep them all busy, but there isn't enough person-power to analyze and interpret all that data. So if one has money to build 10 JWST's, a likely better use of that money would be to build one or two and spend the rest of the money hiring people (graduate students, postdocs, faculty) to do science with the data. Or just build one JWST and use the money for a second to build something else that pushes capability in other areas (e.g., the "Origins Space Telescope" far-infrared mission concept, or the Lynx X-ray telescope concept). I suspect that would be better for how much new science you can do per dollar spent.