A few figures are trickling in.
A Tomahawk cruise missile--we've expended more than 50 so far--runs about $1.5 million. The aircraft we're using in Syria and Iraq--F-22s, F-16s and B-1 bombers--"cost between $20,000 and $65,000 per flying hour." We now have approximately 1,600 military advisers "authorized" for Iraq; the Pentagon says more could be needed. "A single soldier [represents] $1 million per year."
A former national-security budgeteer "estimates the fight against ISIS could cost between $15 billion to $20 billion annually."
The Bush administration initially estimated the total cost of the Iraq war at $50-$60 billion. It was off by a factor of at least 20. The costs are still mounting.