The United States has spent about $1.1 billion since mid-June on military operations in Iraq, including the more recent airstrikes in Syria, according to the Defense Department. That works out to roughly $7 million to $10 million a day – a drop in the DOD bucket compared with the hundreds of millions of dollars it spent daily fighting in Afghanistan in 2013.
The Center for Strategic and Budgetary Assessments, a Washington think tank, recently issued an estimate of the cost of operations against ISIS that ranged from $4 billion to $22 billion a year, depending on the duration, scope and the extent to which ground forces get involved.
President Obama has been adamant that the U.S. will not deploy ground troops to Iraq beyond the 1,600 to 2,000 soldiers who will be there for security and to protect U.S. interests. Obama has said his goal is to arm and train "moderate" Syrian rebels to help combat ISIS in Syria as well as somehow bolster the Iraqi government's rag-tag army.
However, House Speaker John Boehner and other congressional Republican leaders are skeptical that Obama's strategy can work without substantially more resources – including more ground troops.
Moreover, Army Gen. Martin Dempsey, the chair of the Joint Chiefs of Staff, testified on Capitol Hill last month, "If we reach the point where I believe our advisers should accompany Iraqi troops on attacks against specific [Islamic State] targets, I'll recommend that to the president."
An analysis of the Center for Strategic and Budgetary Assessments, a "lower-intensity air campaign" would cost between $2.4 billion and $3.8 billion a year, a "higher intensity air campaign" would cost $4.2 billion to $6.8 billion, while an all-out "boots on the ground" campaign would cost $13 billion to $22 billion. That third option assumes 25,000 U.S ground personnel would be deployed to Iraq and Syria.
"This force is assumed to consist of several thousand special operations forces at the 'tip of the spear,' supported by a combat aviation brigade, two brigade combat teams, and other forces providing logistical and medical support, all based in Iraq and/or Syria," the report stated.
The Wall Street Journal reported earlier this week that ISIS "appears to have largely withstood the airstrikes so far and with scant pressure on the ground in Iraq and Syria, the militants have given up little of the territory they captured before the campaign." Moreover, the Pentagon has said that airstrikes alone cannot save the ISIS-besieged town of Kobani, a strategically vital area along the Syria-Turkey border, The Washington Post reported yesterday.
Read MoreWhy the US should take PKK off the terror list
Some experts are cautioning that the U.S. may be stumbling into another long-term conflict in the Middle East with huge budgetary implications, just as it did a dozen years ago following the 9/11 attacks on the U.S.
A study by a Harvard researcher last year estimated that the U.S. wars in Afghanistan and Iraq will ultimately cost taxpayers a startling $4 trillion to $6 trillion when "taking into account the medical care of wounded veterans and expensive repairs to a force depleted by more than a decade of fighting," The Post reported.
Linda J. Bilmes, a public policy professor at Harvard, found that the U.S. has already spent about $2 trillion for the military campaigns in Afghanistan and Iraq. Those costs, she said, "are only a fraction of the ultimate price tag."