I assume you mean cost/J or cost/MWh. Construction and maintenance will be most of that. Most credible fusion reactor designs product a lot of neutrons, and neutrons get absorbed by most materials and degrade them. (And turn some materials radioactive — avoiding long term radioactivity is an important design considerations.)

There will also be staffing costs, fuel costs (small, but still — deuterium and tritium aren’t free), and even costs associated with obtaining cooling water.

The big benefits of fusion over fission, as I see it, are that the fuels are safe and plentiful, the byproducts are harmless (if neutron activation is well managed), and the reactor itself isn’t full of extremely dangerous materials. If you turn a fusion reactor off, it’s off, and there’s no risk that it accidentally keeps reacting.