The thing is that these numbers are big for us and most companies, but these are government numbers and decades of work. Both factors are important. It's $11bn over 24 years, or 458m/yr. I'm happy to pay an extra $3/yr for this project.
Yes, I think many of us would be happy to. But the reality is that NASA has to fight a pretty ferocious budget battle every year, and often (not always) delay and increase in JWST cost has meant postponing or cancelling other missions.
For example the Nancy Grace Roman Space Telescope, which was deemed the top priority by the decadal survey in 2010 almost got canceled in 2018/2020 because of JWST overruns.
This Nature article is a good read: "The telescope that ate astronomy" [1] (and at the time of the article, JWST cost was "only" at 5B$).
I am very excited about what we will observe with the JWST, but shelling yet a few other billion of dollars out of NASA's tight astronomy budget to get an identical copy, mean we are yet again cancelling or postponing other exciting missions that could help us answer very important questions.
Perhaps the cost of NASA probes is so high because politically they cannot tolerate failure. This drives the cost up 10x, which means it's a self-fulfilling prophecy that failure is career-ending.
Contrast this with Musk's approach to blow them up until they work, and then he has cheap launch vehicles.
Well by comparison, here is the cost of two of the most impactful recent-ish (post 2000) space telescopes the US has launched:
- Spitzer: 700M$, JWST being it's successor. This telescope allowed us to detect an exoplanet through light for the first time, refine our understanding of the shape of the milky way, find candidate objects to be further observed by JWST and many more contributions.
- Kepler: 600M$, this is the telescope that allowed us to understand that planets were not rare at all, detecting more than 2500.
So imagine what you can do with 3 to 5B$. Certainly more interesting things than just doubling your data gathering rate of a single telescope.