Finally - one must actually consider the other costs. Even if we ignore human lives (which no sane person would actually do - but we'll leave them out since they are hard to calculate), climate change is already causing refugees and displacement.
Look at the impact that just a few million refugees have had on Europe and the USA in recent years - politics have gone completely off the rails, there are days we seem to be on the verge of a world war, xenopobia has skyrocketed and compassion appears non-existent. 
Now imagine what happens when you have a billion of them ? That's likely on even the most optimistic outcomes of 2 degrees warming, can you imagine the strife, poverty, violence and political upsets that would follow as the world tries to accomodate them ?
Last I checked strive, violence and political upsets tend to harm the economy - a LOT. It's not even remotely possible that the costs of that would not, by itself, exceed the cost of replacing almost all fossil fuels.