This is straightforward: We've spent more than 5 trillion on the war against terrorism, homeland security, NSA, and other measures as a response to 9/11.
If you want to talk money and not lives, then by your citation we're spending the equivalent economical impact of a large terrorist attack every month to prevent something that is more rare than being struck with lightning from happening.
That's exactly my point. It would have been better to spend large amounts of money preventing 9/11 from happening so we wouldn't have spent the 5 trillion we did on our reaction to it (never mind the cost it had to our national psyche). If someone hits a skyscraper in NYC or the Pentagon with what is effectively a cruise missile, the US military is going to be on the ground to get the bad guys within the month and we'll stay until we find them. That's just a law of nature; its what the people demand from their government, not just in America but in general. Given that, it makes a lot of sense to spend money on preventing terrorism. The direct and indirect costs of a successful terrorist attack are huge.
http://nationalpriorities.org/cost-of/
If you want to talk money and not lives, then by your citation we're spending the equivalent economical impact of a large terrorist attack every month to prevent something that is more rare than being struck with lightning from happening.