For technical reasons deep space missions usually run on Plutonium 238 RTGs. Pu-238 is scarce: "In 1992, the US arranged to buy 30 kg of Pu-238 from Russia for

For technical reasons deep space missions usually run on Plutonium 238 RTGs. Pu-238 is scarce: "In 1992, the US arranged to buy 30 kg of Pu-238 from Russia for US$6M ($200K/kg). Even counting the effects of inflation, this was a real bargain, although in the end only about 20 kg was delivered." (Source: https://newatlas.com/nasa-cancels-advanced-sterling-radioisotope-generator/29880/ https://newatlas.com/nasa-cancels-advanced-sterling-radioiso...). More recent proposals to restart US Pu-238 production using a dedicated reactor suggest a probable price of $10-15M/Kg, meaning that a single RTG for one deep space mission will require on the order of $100M of fuel and soak up 4-8 years national production. A further ramp-up of Pu-238 manufacturing is not inconceivable, but with PV cells that can operate usefully in Jupiter orbit, we only need Pu-238 for outer planet missions and some specialized purposes (e.g. a Mercury lander that spends a lot of time in complete darkness, or a truck-sized Mars rover).