"Although the term 'homeland security' existed before 2001, the U.S. Commission on National Security/21st Century was one of the first entities to comprehensively examine the concept as a primary mission of the federal government, just months before the terrorist attacks of September 11, 2001. Since those attacks, the federal government has spent more than half a trillion dollars in its efforts to prevent further acts of terrorism in the United States and to prepare government at all levels, as well as private industry and the general public, to respond to such attacks if they occur. Funding for such activities increased substantially in the immediate aftermath of the attacks, when lawmakers enacted the Homeland Security Act of 2002 (Public Law 107-296). That legislation, which resulted in the largest reorganization of the federal government since the National Security Act of 1947, created the Department of Homeland Security (DHS) by combining 22 separate agencies into a single entity with principal responsibility for homeland security. Over the past decade, all other Cabinet-level departments, as well as a number of independent federal agencies, have also received funding related to homeland security, and most continue to receive some funding for that purpose each year. For 2012, lawmakers allocated a total of $68.0 billion for homeland security, 1.5 percent more than the funding provided for 2011. The President has requested $68.9 billion in funding for 2013, 1.3 percent more than the 2012 allocation. About 90 percent of that 2013 funding would be concentrated in four departments: DHS, the Department of Defense (DoD), the Department of Health and Human Services (HHS), and the Department of Justice (DOJ). The remainder of homeland security funding is spread among the 11 other Cabinet-level departments and 16 agencies."
Congressional Budget Office: http://www.cbo.gov/