I think a good way to look at it is "How many chests full of U238 should I make the player stockpile before unlocking Kovarex?" because that's the true limiting factor here. There are no ways to spend the U238 before doing nuclear research. A full steel chest of U238 (4800) will yield about 34 U235. So, if you believe the player should be stockpiling 10 full chests, then a value around 350 is reasonable.
Personally, I think anything beyond 2 or 3 chests is excessive, because there isn't any gameplay benefit to it. There are no puzzles for the player to solve, other than "build more chests and wait".
Perhaps an alternative would be to provide a "Simple Kovarex enrichment process" tech that gets unlocked easily by trigger but is less efficient, and then the real fully efficient Kovarex tech can keep its original expensive cost. This is how Space Age handled coal liquefaction on Vulcanus, so there's precedence for the approach.
(I like the Kovarex unlock options as they are right now. I don't think you actually need to change anything - just wanted to add to the discussion here)