How is the cost of an ecosystem service, such as water filtration performed by a wetland, calculated? It's based on how much it would cost to replace the service with existing technology. Engineers build an artificial wetland in the lab and run it for a few years; how much that all costs to do is the cost of the ecosystem service. Scientists create elaborate computer models that do the cost estimating. It's none of these answers; the cost estimate is really just a best guess on how much clean water costs