Actually you can come close to a cost of 9/11, and you can add to it the cost of the policy mistakes made in its name.
As it was we were not "doing nothing" we went to Afghanistan, attackend and broke up the group responsible for 9/11 and then inexplicably let the mastermind escape to sanctuary in Pakistan.
We attacked the perpatrators and addressed the issue. That's not "doing nothing."
Iraq had nothing to do with it. It's a boondoggle tacked on and justified by 9/11. Thankfully for the government most voters can't even locate Iraq on the map, let alone work out that they didn't have a connection to 9/11.