It's cheap because we are offsetting the cost if its ultimate pollution onto future generations. We do this for everything else, and nuclear is our best chance for a liveable planet - if we don't want to make the slightest effort to give up on our comfort. But we have the belief that humanity will be able to manage nuclear waste for the next 100k years while we don't know how the pyramids were built... and it was only 3k years ago.