Compared to the expense of failed crops? Mass migration of all humans to the northern most reaches of Canada?

I really don't think you get it. If temperatures rise enough, most of Central and North America won't be uninhabitable. The parts that aren't underwater will be too hot and parched. We won't be able to grow crops or desalinate enough water when the clouds and rain disappear (yes that will happen).

The 3 days of intense heat here in Portland left plants all around my yard scorched and half dead. Just 3 days. Imagine that happening regularly around the world.