The cost is a hiccup to our GDP. Vietnam was 12% and Iraq is only about 2%. It is still an if but when Iraq succeeds and becomes a prosperous nation and a friend in an otherwise terrible place the cost will be well worth it.
If you want to compare costs look at what NASA waists every year. Look at what our government spends on foreign aid, AIDS in Africa, Global warming research, etc... Granted I'm all for all of the above but if you are going to make the "wouldn't that money best be spent somewhere else?" argument, you have to look at everything.
Unfortunatly we have to wiegh human life. How many Iraqis would have been planted if we left Iraq alone. How many Americans and others around the world would or could end up dead if Saddam got a nuke and sold it to a terrorist? Look at Saddam's 30 year history of torture, rape, and murder. He holds a body count of somewhere in the millions. How many people are him and his wacko sons going to kill now?