Actually, they aren't; even at peak numbers of deployed and stored weapons, the total budget for nuclear weapons (including R&D, acquisition, and O&M costs) never exceeded 1% of the DoD budget, and we had something like 30,000 weapons in the stockpile--and that was at grossly overpriced government rates.
And many of the folks interested in nuclear weapons would not be interested in holding onto said weapons long enough for maintenance to be an issue.