DEMO is not a singular reactor, but rather a class of reactors that are expected to be built using the technologies and lessons learned from ITER. So basically ITER’s main goal is to be a massive international R&D project to pave the the way for individual countries to be able to build their own DEMO plants afterwards. Pathfinding manufacturing technologies for this unique of a system is a big part of why ITER is so expensive. Replicating an ITER like machine should be considerably cheaper.