The economic impact depends mostly on the loss of homes and other buildings, not farmland. A good fraction of the world's wealth is tied up in buildings, and those tend to be concentrated near coastlines. But levees can be built in many areas, and some buildings in other areas might be worth putting on stilts. So the costs of levees, stilts, and abandoned structures are what you most need to examine, I think, to assess the economic cost of sea-level rise.