I am not an economist, so I would argue that the whole premise is flawed: you can't measure the value of an ecosystem in dollars. You can't eat gold. Money is some sort of abstract concoction, whereas plants and animals are very real. No amount of money would make it alright to destroy them (the paper argues a different point, but still thinks of them in terms of dollars).
Saying you can't measure value in dollars is kind of like saying you can't measure a distance with inches. Inches are also a kind of abstract concoction.
Inches are a defined construct, not an absolute. They are convenient chunks so we dumb humans and communicate with some level of accuracy.
Dollars are exactly the same - assigning value lets us communicate and relate otherwise abstract concepts in a meaningful way. While we can't eat gold, we can imagine how much food gold can buy, with all the caveats about exchanges and locations in place. The important bit to take away is providing a framework for common understanding.
