His assertion came as part of a fiery speech urging world leaders to get their act together against violent Islamic extremists who are taking over ungoverned fringe areas of the middle and far east, and Africa.
Kerry was trying to make the case that united international action against Islamic State / ISIS actually works - the terrorists' resources have been degraded in some areas. But most observers will likely be surprised that Daesh, the new negative nickname for Islamic State that Kerry used repeatedly, has an economic structure featuring wages for its soldiers.
Kerry has said this before. In late 2014, Foreign Policy magazine noted:
The militant group's intake of money from extortion and smuggling is dwindling and that's hurting their ability to pay their fighters, Kerry said. "We've already seen a 75 percent cut in pay for ISIL fighters in Mosul."
An entire economy has grown up around Daesh, and it's oil based. Bloomberg reported in August that Daesh may be getting "more than $2 million a day in revenue from oil sales, extortion, taxes and smuggling, according to U.S. intelligence officials and anti-terrorism finance experts."
For a fighter on the ground in Iraq, that translates into $1,000 a month - a decent sum for Iraq, as FP notes:
The coalition has managed to shrink ISIL's oil revenues from about $1 million a day to a few million dollars a week, David Cohen, Treasury Department under secretary for terrorism and financial intelligence told U.S. lawmakers last week.
"We have information that they pay their fighters about $1,000 a month," Cohen said. "That comes to about $360 million a year in just the expense for fighters."
Looks like some of those fighters are down to about $250 a month, if Kerry's info is still current.
Kerry elaborated a little on the way Daesh uses money as a recruitment tool. "They grab these young kids, pay them a little bit and brainwash them. Then they don't have to pay them anything," he said in Davos.