The cost of the land upon which the nuclear power plant (NPP) is built.
Costs related to designing the NPP
Cost related to the materials from which the NPP is built.
Labor costs related to manufacture and construction.
The cost of obtaining regulatory approval
Investment related costs (interest, etc.)
Transportation and Access related costs
The cost of the electrical transmission system that connects the NPP to the grid
Any attempt to lower nuclear costs must find a way to lower the cost of one or more of these.