"To continue to field a nuclear force roughly the same size as it is today, the United States plans to modernize virtually every element of that force over the coming decades. The Congressional Budget Office [CBO] estimates that the most recent detailed plans for nuclear forces, which were incorporated in the Obama Administration's 2017 budget request, would cost $1.2 trillion in 2017 dollars over the 2017-2046 period: more than $800 billion to operate and sustain (that is, incrementally upgrade) nuclear forces and about $400 billion to modernize them. That planned nuclear modernization would boost the total costs of nuclear forces over 30 years by roughly 50 percent over what they would be to only operate and sustain fielded forces, CBO estimates. During the peak years of modernization, annual costs of nuclear forces would be roughly double the current amount. That increase would occur at a time when total defense spending may be constrained by long-term fiscal pressures, and nuclear forces would have to compete with other defense priorities for funding. In its first few months, the Trump Administration began a new Nuclear Posture Review to determine a nuclear policy and force structure 'appropriately tailored to deter 21st century threats.' That review may recommend changing modernization plans and force sizes inherited from the Obama Administration to reflect the Trump Administration's priorities for nuclear forces or to shift resources to address other defense priorities in the face of long-term budgetary pressures."
Congressional Budget Office: http://www.cbo.gov/