It's not only initial investment. Half of the fusion fuel is tritium, which is one of the most expensive substances on Earth (a google search finds that the price of tritium is about $30k per gram [1]). For comparison, fission reactors need enriched uranium, and that costs only about $4000 per kilogram [2]. People have the idea that fusion produces many times more energy than fission, probably because fusion bombs have a higher yield than fission bombs. This is not true. The most typical fusion reaction involves one deuterium and one tritium and yields 17.5 MeV from a total or 5 nucleons. A fission reaction involves one neutron and one atom of U-235 and yields 190 MeV from 236 nucleons. So fusion yields about 4.3 times more energy per nucleon. That's respectable, but in the popular imagination fusion yields 100 or 1000 times more energy than fission, so the fuel cost can be neglected. Nothing could be further from the truth.
It's not only initial investment. Half of the fusion fuel is tritium, which is one of the most expensive substances on Earth (a google search finds that the price of tritium is about $30k per gram [1]). For comparison, fission reactors need enriched uranium, and that costs only about $4000 per kilogram [2]. People have the idea that fusion produces many times more energy than fission, probably because fusion bombs have a higher yield than fission bombs. This is not true. The most typical fusion reaction involves one deuterium and one tritium and yields 17.5 MeV from a total or 5 nucleons. A fission reaction involves one neutron and one atom of U-235 and yields 190 MeV from 236 nucleons. So fusion yields about 4.3 times more energy per nucleon. That's respectable, but in the popular imagination fusion yields 100 or 1000 times more energy than fission, so the fuel cost can be neglected. Nothing could be further from the truth.
[1] https://www.google.com/search?q=tritium+price
[2] https://www.uxc.com/p/tools/FuelCalculator.aspx