"Astronomical" isn't an actual number. You still have to decide if it's a billion dollars or a trillion dollars or a billion trillion dollars. It's possible, and harmful, to choose a number which is too large.
I find myself agreeing with you and at the same time thinking that your position is absurd. Let me try to put this into words. I might be missing parts of your point.
If we measure out the cost of avoiding total environmental collapse - which also implies human extinction - and we put a finite number on it, we've said 'the survival of human life on earth is worth this much and not more'.
It's really hard for me to get my head around the idea that we would find a dollar value that has any meaning that would properly describe the limit of how much humans value their own survival. I wouldn't know where to start, what to measure, or what to exclude. I'd personally be loath to excluding anything, as a fan of existence.
I'm not convinced I'm right to think putting a dollar value on existence is folly -- but I can't conceptualize what it would mean.
