I a previous post I was surprised that natural disaster, not including climate change, could amount to $185 billion a year by 2100 and climate change would be much less at $28-$68 billion annually. This was reported in Scientific American. But the climate costs didn't include other factors such as desertification or rising sea levels.
Now another report also in Scientific American suggests that costs have been underestimated and tries to nail down what the costs of Climate Change could be. There are still a lot of uncertainties in this but an inter-agency workgroup's best guess of what each ton of carbon dioxide dumped in the atmosphere costs is roughly $121 billion worth of damages annually. It should be noted that this is American research and these figures only reflect the cost to the US economy. Costs globally I imagine would be much, much more.