> 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.

[1]: https://www.nature.com/articles/4671028a

This isn't so much a limited resource problem as an allocation problem. If US, and western, interest increases in science then funding more missions like these will be more popular and Congress will increase budgets. But budget allocations are low because they are unpopular. We talk about the high costs of these missions but they are close to rounding errors in the US yearly budget. It's weird to me that we argue over pennies when we're spending hundreds with no question. We'd say that's insane if we were trying to set budgets for ourselves. The reason it works it because the total numbers are large and we aren't scaling properly. It's a distraction.

If it makes you feel any better, if I had Gates' money I'd be funding a fleet of probes. Most of the same design :-)

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.