it's insanely costly _to build bring a plant online initially_
once you're up and running, it's cheap as dirt, just takes decades to pay off the initial investment
10MW is insanely costly, regardless. That's so small that it isn't enough to justify even the overhead of running a plant. 1GW or so is the smallest size where it starts to make sense to design a nuclear plant that has a hope of delivering electricity at a price that matches the market (and even then: usually only by subtracting decommissioning and construction costs from the equation).
for the plants that are being decomissioned, those costs had already been paid off! that's the whole point; the ongoing costs for these plants were low, so it is the height of derangement to shut them down at the very moment germany needs reliable low-carbon locally-sourced energy. decomissioning them is monumentally expensive and amounts to civilization-scale vandalism.
It's dirt cheap because it's heavily subsidized by the public. For example no commercial entity in the world insures a nuclear power plant. These costs are completely hidden.