It would cost a fuck ton! Here's how I figure it:

Each fusion event makes one He atom and releases about 17 MeV of fusion energy. Since (one mole) X (one eV) = ~100,000 Joules, fusing one mole (2 grams) of He would produce about 1.7 X 10^12 Joules. That's about 472 megawatt-hours of fusion energy produced.

Another commenter says that the energy INput (to the Farnsworth fusor) is about 100,000 times the energy OUTput; so fusing that 2 grams of He would require a 2,000 megawatt power plant to run for 23,600 hours, or 2.7 years.

Readers: please correct any mistakes you find.

> fusing one mole (2 grams) of He would produce about... 472 megawatt-hours

Or, from a different angle, that energy density is why fusion is the holy grail of energy.

Since a fusor normally runs on deuterium fuel, wouldn't the output be helium-3 with a mass of 3 grams/mole?

Not that it makes a big difference - producing helium with a fusor remains extremely impractical.

Oops! I used He's atomic number (2) instead of its atomic mass (usually 4).

I believe deuterium fusion events mostly produce He4, occasionally He3 + neutron. So we'd actually produce between 3 and 4 grams.

No, deuterium-deuterium fusion reactions usually produce helium-3 and a neutron. Due to the conservation of energy, producing helium-4 requires the emission of a gamma ray. This happens rarely because, since the strong nuclear force is stronger than the electromagnetic force at small distances, fusion reactions tend to release energy as protons and neutrons rather than gamma rays.