It depends on who's point of view we're evaluating it from. I haven't looked, but suppose there is an endangered species which call the lake home, perhaps that is the only place they inhabit. Putting them one oil spill away from existence.

So, what's the going rate for pristine lakes nowadays?

It’s actually really high since you’d be ruining the enjoyment of the lake for all future generations plus the fact that you can’t predict how much enjoyment they would get from it.

But if there were enough benefits it could still be worth it. Brining 50k people out of poverty for example? Reducing trucking pollution by a large amount.

Best I can do is about tree fiddy.