May be you mean uranium enrichment? When plutonium is already available, like in Ukraine, it is completely different game - you just need to chemically separate plutonium from the spent fuel. That is the only noticeable expense - needs facility with safety protections, etc. Or just a few people willing to die for the Motherland. The rest is putting charges around the plutonium sphere and timing them correctly with electronics. That is basically all. Couple weeks really :)