If true - how fast could a new transmission line from France to Germany pay for itself?
Germans would get the full cost which is much higher than the quoted price here. Besides nuclear power is not flexible enough to amp it up and down according to demand (unlike coal or petrocarbon industries).
TLDR More interconnector capacity, battery storage, and renewables needed (my analysis).
[1] https://www.oecd-nea.org/upload/docs/application/pdf/2021-12...
[2] https://app.electricitymaps.com/zone/FR?wind=false&solar=fal...
[3] https://www.euractiv.com/section/coal/news/france-extends-li...
But the way these networks work, you can often build up the interconnects where there's a pricing/availability disparity, and pretty soon you have a vast interconnect. Because if Germany's grid pricing benefits from relatively unconstrained interconnection with France, there's going to be a pricing disparity at Germany's other borders. Of course, grid borders don't necessarily reflect national borders, and national interconnection projects have to happen too.
That said, the top 5 countries by peak load in wikipedia are France, Germany, UK, Italy, and Spain, which are all either France or neighbors of France.
[1] https://en.wikipedia.org/wiki/Synchronous_grid_of_Continenta...