The creation of the new cabinet-level Homeland Security agency after 9/11 and the associated identification of a new homeland security mission to prevent, respond to, and recover from terrorist attacks, constituted the largest reorganization in the United States government since World War II.
Absent the wars, the US would have spent some money on the areas we now identify as homeland security, but nowhere near as much. Between Fiscal Year 2001-2016, homeland security appropriations are estimated to be $548 billion higher than they would have been otherwise.
State and local governments have also spent money on homeland security, but it is difficult to assess how much of that spending has been reimbursed by federal grants. A 2003 US Conference of Mayors’ survey estimated that cities nationwide were spending $70 million per week more on homeland security than they had before 9/11.
If federal spending on homeland security had grown at the same rate as other (non-military) federal spending, total appropriations would have been hundreds of billions of dollars lower.
(Page updated as of September 2016)