France does run mostly on nuclear, but the less money claim needs some serious evidence. The reason why France can export energy more cheaply is because the cost of nuclear is pretty much all capex, so now that they have their plants it makes sense to constantly (nuclear plants can't easily be adjusted to load) run them at a high output and export excess energy. However, the overall cost is very likely much higher (the main reason why nobody wants to build new nuclear plants). France nuclear industry was (and still is?) heavily subsidised because nuclear capability (in particular military) is regarded of strategic importance.