We have no idea what those costs will be. Given that for-profit companies are trying to develop new fast reactors, at least some people think it will actually be profitable.
However, I guess it's reasonable to include some such estimated cost in today's reactor prices, if we also:
1) Include the costs of climate change and/or ambient carbon removal in the cost of all fossil fuels, and
2) In wind/solar costs, include the cost of all the batteries, long-distance transmission, demand management, and overproduction that we'd need to keep the grid reliable without the fossil backup we're using today. Areas with sufficient hydro can skip this.
However, I guess it's reasonable to include some such estimated cost in today's reactor prices, if we also:
1) Include the costs of climate change and/or ambient carbon removal in the cost of all fossil fuels, and
2) In wind/solar costs, include the cost of all the batteries, long-distance transmission, demand management, and overproduction that we'd need to keep the grid reliable without the fossil backup we're using today. Areas with sufficient hydro can skip this.