U.S. has already spent nearly $1 billion fighting ISIS - Sep. 30, 2014

The first wave of U.S. military attacks against ISIS has cost nearly $1 billion, according to a military think tank.

And costs could rise to as much as $1.8 billion a month if the U.S. military presence grows to 25,000 ground troops, as some have suggested, said the Center for Strategic and Budgetary Assessment (CSBA).

U.S. has already spent nearly $1 billion fighting ISIS - Sep. 30, 2014