> why can't we spend less than that to avoid the problem

We can, and we are. That's why I do what I do.

You're absolutely right, it is much cheaper to avert climate change than deal with the consequences. It's also necessary to transition to sustainable energy eventually because we will run out of fossil fuels regardless of what happens with the climate.

The solutions to climate change are being worked on right now by many talented people around the world. I'm confident we will reach the IPCC SR15 1.5 degree Celsius goals.