Letter from CBO Director Dan L. Crippen to the Honorable Thomas Daschle Regarding [Potential Costs of National Missile Defense Systems] [open pdf - 255KB]
From the Letter: "The cost estimates that CBO [Congressional Budget Office] has prepared for individual systems should not be added together to yield an estimate of the total potential costs of national missile defense. For example, the sum of the estimates for the ground- and sea-based midcourse missile defense systems does not reflect the costs of a single combined system. If they were part of a combined system, the ground- and sea-based systems could draw on some of the same research and development activities and share some of the same sensors, command and control facilities, and components. In addition, as you requested, CBO's assumptions about the architecture and components of the sea-based system reflect its use as a stand-alone system, not as an adjunct to a ground-based system. Finally, in many cases substantial uncertainty exists about the relationship between the system descriptions available to CBO and whatever missile defenses might ultimately be deployed as a result of the programs being planned by the Bush Administration."
Congressional Budget Office: https://www.cbo.gov/