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…$100 billion in lives, health, property, and broader economic damage. Including the longer-term policy response, roughly $1 trillion in homeland security spending and another $1 trillion for the war in Afghanistan can reasonably be attributed to the attacks.
More debatably, one might add another $1 trillion or more for the war in Iraq. Other attempts to quantify the broader costs of the post-9/11 security state and counterterrorism have gone much higher. A from Brown University’s Costs of War Project, for example, assessed the total budgetary impact at $8 trillion.

Quantifying in dollar terms the toll taken by a horrific event and its many consequences