Costing disaster relief is certainly something that would be possible. One would need to look at various government departments and, in fact, at various levels of government.
The difficulty would probably lie in determining which part of that disaster relief is due to climate change and which part is due to naturally occurring events—events that would have occurred anyway, without climate change—and also the part that is due to people choosing to live in certain more risk-prone areas, which is also a factor with cities expanding and people choosing to live in certain areas. That's where most of the difficulties would be, but it certainly is something that is possible. It wouldn't be a perfect exercise, but it's possible to do that.