In 1965 the Atomic Energy Commission put the cost of plutonium 239 at $10/gram, which by simple consumer price indexing (only a rough guide, admittedly) would m

In 1965 the Atomic Energy Commission put the cost of plutonium 239 at $10/gram, which by simple consumer price indexing (only a rough guide, admittedly) would make it $96/gram today. See Table I in this publication "Values In Spent Fuel From Power Reactors": https://www.osti.gov/servlets/purl/4618921 https://www.osti.gov/servlets/purl/4618921 At $96 per gram instead of $4000, that's a much more attractive $154 million to put 1600 tons in orbit. Uranium enrichment costs have fallen greatly since the 1960s as centrifuges have replaced gaseous diffusion, so uranium-fueled units would probably cost less (despite the greater mass of uranium required). I'd say that the main modern problems of a terrestrially launched Orion are treaty violations and giving a plausible cover for nations to again pursue developing "peaceful" nuclear explosives.