Under the Bush administration, in 2002 the United States withdrew from the anti-Ballistic Missle Treaty and began to pursue an ABM system. According to a Congressional Budget Report, the total estimated expense of the system by 2024 could be as high as 247 billion USD. In the Cold War, an ABM system would have been quite an advantage given the Nuclear Missile Technology of the 20th Century. I put forward the idea that if a legitimately recognised Nation State possessing nuclear weapons did wish to deliver them to a target, 21st Century technology including unmanned subs and UAVs will make the delivery of such devices far easier to deliver than launching a Ballistic Missile. Even using 3000 year old technology such as shipping the nuclear device covertly in a standard shipping container would have a high probability of success. Subverting an advanced technology is always much cheaper than the technology's development.
A regular criticism of the US military is that they are constantly fighting the previous war rather than looking ahead. The money spent on Missile Defence will be completely wasted. The United States in developing this technology continues to alienate its allies, a cost which is high but difficult to quantify. Imagine if instead the $247 billion were either never spent at all or rather spent on basic scientific research grants or technological competition such as the DARPA Challenge.