The easy answer to that question is about 3 Trillion dollars since 9/11. To put that in perspective approximately 2989 people died in the attacks, some reports say more, some say less. No other people have died within our borders from acts of terrorism since. It has been just under 10 years and when you average the numbers out it works out to about 300 people a year. Flip the coin back to the cost, 3 trillion divided by 10 is 300 billion divide that by 300 people works out to a BILLION DOLLARS spent per person killed per year for 10 years. More people are killed crossing the street each year then this.
The other way we have paid is with our privacy and personal freedoms. The Patriot Act, Homeland security, The TSA, and other existing government entities that have been beefed up and all of them are protecting you for your own good whether you need/want it or not. So, the next time you are being groped by a TSA agent or your privacy is being trampled on by the feds remember the more freedom you give away in the name of security, the deeper the government will delve into your everyday life.