War against ISIS already costs nearly $1 billion

WASHINGTON -- The air war on ISIS have so far cost the U.S. nearly $1 billion.

The Center for Strategic and Budgetary Assessments, a military think tank, has listed a range of estimated costs to fight the Islamic militant group.

The study suggests the cost to maintain the same level of military air operations totals about four billion dollars a year.

The addition of 25,000 troops on the ground could cost $1.8 billion a month, but President Obama says he has no plans for U.S. troops to engage in combat.