all it says is that the US is willing to suffer more casualties in revenge attacks that it suffered in the first place, which isn't a very meaningful cost measure. It's also based on political denial - to most people outside the US (and hopefully many inside) the WTC attacks were an excuse not a reason. The reasons for invading Iraq already existed.

A more meaningful attack is the number of days it took the US to kill more innocent civilians in revenge than died in the WTC attacks. I beleive the number is about 90, but I could be wrong (it also depends on what you count, that one is direct military action rather than the usual starvation and deaths from "sanctions").