Once you’re already enriching uranium to the quality necessary for nuclear fission, half of your work is already done for you if you want to enrich it further, up to weapons grade.
Edit: Furthermore, “Space Program” should reduce the costs of “Nuclear Weapons”, as once you are already investing in precision rocketry for exit and re-entry, most of your work is already done for you, if you want to put nuclear warheads on your rockets.
I don’t think that the “Nuclear Weapons” program costs are much of an issue, as they currently stand in the game. US Nuclear Weapons spending is about 8-10% of total military spending, which the game models accurately. However, I suggest that the base cost be raised, and then lowered back down to what it is now, if there is no “Space Program”.
As for “Nuclear Fission” - the costs of real-world Nuclear Weapons programs are lower for countries that, once upon a time, had a nuclear fission program. However, decommissioning those programs, in the present day, does not cause those governments to forget everything they learned about uranium enrichment. So, while it is historically true that it is a cost-savings measure to have once had that technology, it may not be true that there is a present effect.
For every country other than the United States, this does not work in exactly the same way, as these countries may be able to just buy or steal American ballistics technology without having their own domestic space program, as in the case of China’s recent acquisition of Russian hypersonic missile technology, or Israel’s entire nuclear weapons program.