I'm not saying your conclusion isn't wrong, but your argument probably is - largely because there are many costs of terrorism beyond simply loss of life - it's an incredibly efficient way to disrupt a nation.
For example, some estimates of the cost of 9/11 (I think including the costs of Iraq and Afghanistan) suggest that each American is out to the tune of $10,000 [0]. If the NSA were able to prevent one 9/11 style event, they'd in one step justify their budget for 300 years (if you ignore Iraq and Afghanistan, it goes down to 75 years, but in many ways at least Afghanistan was very much caused by 9/11).
For example, some estimates of the cost of 9/11 (I think including the costs of Iraq and Afghanistan) suggest that each American is out to the tune of $10,000 [0]. If the NSA were able to prevent one 9/11 style event, they'd in one step justify their budget for 300 years (if you ignore Iraq and Afghanistan, it goes down to 75 years, but in many ways at least Afghanistan was very much caused by 9/11).
[0] - http://www.nytimes.com/interactive/2011/09/08/us/sept-11-rec...