According to a new report by the Carnegie Endowment for International Peace (brought to my attention by Steven Aftergood), spending on US nuclear weapons infrastructure and related programs surpassed $52 billion in 2008. "That's a floor, not a ceiling," said study co-author Stephen Schwartz, who noted that the figure does not include costs associated with classified nuclear weapons or intelligence-related programs.
A view of the spending breakdown, provided by Carnegie:
To put this in some context, nuclear weapons expenditures accounted for some 10 percent of all defense spending... and dwarfed the entire federal budget for "soft power" programs like international diplomacy and foreign assistance, which amounted to just $39.5 billion last year.
Of the $52 billion spent on nuclear programs, 55.5 percent went to upgrading and maintaining the existing stockpiles of weapons, whereas just 10 percent was invested in nonproliferation programs aimed at preventing the spread of such weapons around the globe.