There are plenty of ways to calculate the value of a clean atmosphere, though I agree that a good calculation is difficult, and the question of who should pay for it and how isn't easy either (but more of an ethics-politics question than the value itself).

I disagree that we'd be better off if they assumed the value was infinite. Under that calculus, anything that might possibly impact the atmosphere would be banned: things like energy generation of nearly any form, which most reasonable people would agree is of positive benefit.

There is a value to it, that value is hard to calculate, but unfortunately, 0 is much closer to the real value than infinity.