If $10B is hard (i.e. impossible) to justify for proven fission, how about $100B for fusion? $200B?
Nobody involved expects ever? to field a competitive Tokamak power station. Not in their lifetime, not in their grandchildrens' lifetime. That is not the purpose of the project.
It has a different purpose. Its main purpose is to provide employment for hot-neutron physicists, to maintain a population to be available to draw on for sporadic weapons work. Another purpose it shares with a lot of others, to provide a reliable long-term conduit of public funds to select private hands. The hot-neutron physicists are not getting those $billions, but somebody is.
Every penny spent on ITER is stolen from other possibly viable projects. Imagine how many GW-years could be generated by 2050 by solar and wind paid for out of what is earmarked for ITER.