The fuel costs of the nearest nuclear plant to you is about 5-10% of the total cost.

So.

If that 5-10% was now free, how much would that change the cost of building a nuclear plant?

Nuclear fusion sounds great, but the actual 'turn you into Spiderman' bit is pretty small compared to the....

Water -> Electricity turbines, Cooling towers, power distribution systems, all the major electrical lines running to the plant, all the major electrical bits for routing/voltages/disconnects.

We need nuclear fusion, but it won't make unlimited AA batteries or something crazy.