> If you want to calculate GDP some other way then you will need to decide what millions of prices "should" be and persuade others with your argument.
It's a bit worse than that because cost, value and price are all different things.
The amortized cost of making water available from the tap is pennies per gallon. The value of water is nigh infinite because without it you die. The price is presumably somewhere between those two values, but it could be anywhere -- an efficient water company might charge close to the cost but a robber baron with a hard monopoly on water might charge close to the value. The market only tells you the price, not the cost or the value.
In a lot of cases it doesn't even tell you that. Consider what happens when a person spends time on a hobby. The cost is not zero, the value is not zero, and there is no price. Even more complicated, suppose the hobby is writing free software. Then there is no price (or it's zero, which isn't very informative) but you have value being derived by people all over the world, who may each value it in a different amount.
> Philosophically, even major changes like a carbon tax or universal basic income are basically just patches to get prices somewhere closer to what we think they should be.
A carbon tax is definitely about pricing an externality. A UBI is kind of the opposite. It's basically admitting that we don't know how to value everything correctly so instead of that here's the money, go choose what you need for yourself.