Yes it worth it, even if it was 100 billion dollars per month, it was a vital threat to the USA, to understand why you'll need to understand what really happened, it is not a matter of "Arms of Mass Destruction", "Stealing fuel and petrol", "Democracy", "Terrorism", "Helping Israel" ...

The real reason is to protect the DOLLAR, the same thing for the conflicts with Iran and Russia.

To understand this in details please read this Article written by Rudo de Ruijter : http://www.daxii.com/articles/cost_abuse_and_danger_of_the_dollar.aspx

over the top amount quoted but ...remember we need oil for industry and movement of people and we cannot afford to have the blockage of such a vital resource.
It would be like blocking of a river which we rely on for our needs!
There is the death count which is not a nice side-effect but for the many a few perish!
We thank them...sincerely.