How much did 9/11 cost us? Direct costs were somewhat above $100 billion is the estimate I am familiar with. Were these costs recurrent the overall impact is, I believe, in the range of Posner's estimate for Iraq war. And you haven't yet begun to deal with the problem.
So what about recurrence? Saddam, somewhat like Germany as it left the Wiemar era and transitioned to Hitler, was restrained by measures it was significantly noncompliant with. With again a demonstrated murderer in charge 'a stitch in time saves 9' appeared an appropriate proverb.
More generally, what was the problem in the Arab world? The Palestinians had been left by the Arab nations to nurse a grievance. There was no 'Life is about getting and giving up.' They were not to turn to 'get' anything unless it involved direct retribution for their loss. It was thought that this abnormal psychology was more general in the Arab world and promoted scapegoating and was bound up with a lack of liberty. Thus the idea was that liberty would promote 'getting' for the Arab people. This seen would reduce the impetus to scapegoat the US. If you look at the blog Iraq the Model or those citizen journalists in Iraq Michael Yon and Totten linked through Instapundit, you see that the Iraqis are now richer in liberty.