"Need I say more?"

"The White House released budget figures yesterday indicating that the new Medicare prescription drug benefit will cost more than $1.2 trillion in the coming decade, a much higher price tag than President Bush suggested when he narrowly won passage of the law in late 2003."
http://www.washingtonpost.com/wp-dyn/articles/A9328-2005Feb8.html

Probably not.

Kyoto has cost the rest of the world about 350 billion without having any significant result unless creating corruption is a result desired. Had the US signed it, we could have easily spent twice this amount, and again without any result in terms of climate, but a lot more corruption.

Cost of 911:
"Immediately after the attacks, leading forecast services sharply revised downward their projections of economic activity. The consensus forecast for U.S. real GDP growth was instantly downgraded by 0.5 percentage points for 2001 and 1.2 percentage points for 2002. The implied projected cumulative loss in national income through the end of 2003 amounted to 5 percentage points of annual GDP, or half a trillion dollars.
...
Overall however the short-term adverse economic impact of the attacks was far less than feared initially, thanks in large part to good economic crisis management. The Federal Reserve, the Administration and Congress acted quickly to restore confidence, inject liquidity and provide resources to deal with the consequences of the attacks. The Federal Reserve by lowering the price of credit and temporarily providing vast amounts of liquidity helped safeguard the integrity of the financial system and saved many firms from bankruptcy."
http://www.ccc.nps.navy.mil/si/aug02/homeland.asp

So, if you added all cost from 911 it's probably cost us more than GWOT effort- and if the GWOT reduced the chance of another major attack [we haven't had one in the US since 911 {almost 6 years} by say 50% then it saves us potential cost of hundreds of billions dollars.

If we leave Iraq before Iraq can defend itself, resulting feeding frenzy may distract the terrorists for a year or so, but after this and with Afghanistan type base of operation we could expect further attacks upon America itself and of of course nearly immediate attacks on US interests in the region and affect the global economy.