What price freedom?
What was the cost-benefit analysis of World War I for the United States?
However, if you really want something with numbers- let's say that the United States had no business in the middle east and that the first Gulf war wasn't fought.
Saddam now controls all the oil from Kuwait as well as Iraq. Remove that amount of oil from the markets (if Saddam so chose) and figure the impact on the American economy.
Alternately, assume that America backed down at the UN and we did not invade Iraq the second time only.
Your alternative scenarios: a. Saddam does nothing and nothing changes. Plausible? Likely?
b. Saddam chooses to act as an (uninspected now) replacement for the Taliban and Al Quaeda forced out of Afghanistan. Plausible? The enemy of my enemy is my friend. I suspect that Saddam wouldn't mind sheltering someone who tweaks the beard of his enemy the U.S. Do more attacks on the U.S. homeland follow?
c. Uninspected, and now unchallenged by the U.S who withdrew, Saddam might choose to rebuild his weapons program. At some point, he may become a threat to Kuwait, Israel, Iran, your choice. Is there a possible expense to the U.S. if the middle east breaks into a war in any of these areas? Will the flow of oil be interrupted?
d. Libya, and North Korea do not feel threatened. Libya does not disarm. North Korea does not come to the bargaining table. Iraq remains a negative influence in the middle east. Syria is not chastened by the threat of U.S. action
No one, now, can tell what might have happened if the U.S. had not invaded. Any of these scenarios might have occurred, or none. Combinations of them....
I'd ask your friend to give you his scenarios as to what he thinks would be happening now if the invasion hadn't occurred.