The cost of American’s nuclear arsenal is projected to reach as much as $661 billion over the next decade, a new report claims.

The report, released Oct. 7 by Ploughshares Fund, a group that advocates nuclear disarmament, says its estimate ranges from $620 billion, if defense spending stays below inflation, to as high as $661 billion if defense spending keeps pace with inflation.

A closer look at those numbers, though, shows that nuclear costing is more art than science, given factors like the secretive nature of the field, unknowable future costs in fuel or aircraft, and the budgetary whims of Congress.

Last year, the Washington Post fact-checker gave Ploughshares “two Pinnocchios” for estimating that nuclear weapons would cost the United States $700 billion over the coming decade, a figure some nuclear hawks rejected.