So, pulling some numbers out of my rear end, let's suppose that 5% of the world's GDP comes from "land-based" activities in some sense, and let's suppose that losing coastal land is 10% as bad as losing average land (e.g. because most farms aren't right on the coast or near enough it to be badly affected by this change). Then losing 0.04% of the land would reduce "gross world product" by 0.05 x 0.1 x 0.0004 = 0.000002 = 2 x 10^-6 of its value, or about 2 x 10^-6 x $10^14 ~= $0.2B annually. That's a lot of money but seems likely to be a lot smaller than, e.g., the impact on agriculture of temperature changes, or the cost of serious mitigation efforts.
(I'd guess that the costs would be relatively much greater for poorer countries, more of whose economic activity is agricultural.)
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.
Note: Please see this post of mine for more on the project, my sources, and potential sources for bias.
I have written a couple of blog posts on my understanding of climate forecasting, climate change, and the Anthropogenic Global Warming (AGW) hypothesis (here and here). I also laid down the sources I was using to inform myself here.
I think one question that a number of readers may have had is: given my lack of knowledge (and unwillingness to undertake extensive study) of the subject, why am I investigating it at all, rather than relying on the expert consensus, as documented by the IPCC that, even if we're not sure is correct, is still the best bet humanity has for getting things right? I intend to elaborate on the reasons for taking a closer look at the matter, while still refraining from making the study of atmospheric science a full-time goal, in a future post.
Right now, I'm curious to hear how you formed your views on climate change. In particular, I'm interested in answers to questions such as these (not necessarily answers to all of them, or even to only these questions).