When people criticize the Iraq war one of the most common complaints is the amount of money being spent, which is approaching $350 billion as I write this. You can even view a running total if you want. I know there are better ways to spend $350 billion, but the financial cost of the war is a weak argument against the Iraq war.
Criticizing the amount of money being spent on the war implies the Iraq war would be OK if it didn't cost so much money. If you think invading Iraq was a bad idea, ask yourself the following questions. Would invading Iraq be OK if it cost $200 billion instead of $350 billion? How about $100 billion? $50 billion? I bet you answered No to all three questions.
If a war is worth fighting, it is worth spending as much money as necessary to win it. When the British were fighting the Germans in World War 2, I doubt many people in Britain were complaining about the amount of money being spent to fight the Germans. The fact people are complaining about the cost of the Iraq war is a sign the war is not worth fighting, not a reason.