Ah, I was wondering why would anyone tell they can get 5 metric tonnes of gold (~$535M) for 2.5 GW of power (~$500). Regular mercury is ~$210/kg ($1.05m/5 tonnes or 500x cheaper than gold). Although, Hg198 has 10% natural abundance. So maybe they can use raw mercury and still get decent returns depending on what othere isotopes decay into.