ISIS isn't rich—it doesn't even have enough money

Isis is not rich
Isis is not rich

Raids by the Islamic State in Iraq and al-Sham on the Mosul central bank and other smaller banks in Iraq did not provide the militant group with a windfall of hundreds of millions of dollars as has been reported, a senior intelligence official told NBC News.

Mosul mayor and Nineveh Gov. Atheel al-Nujaifi told The New York Times and other news outlets last week that ISIS stole as much as $400 million from the Bank of Mosul and numerous provincial banks, as well as large quantities of gold bullion.

But the Mosul heists provided the group with funds "to the tune of millions of dollars" rather than the "hundreds of millions" that was reported, said the official.

At one point, the Guardian newspaper of London, quoting anonymous intelligence sources, reported that ISIS could be worth $1.5 billion. Intelligence sources highly doubt that figure as well.

Read MoreFriday's oil prices: Crude steadies around $113 a barrel

Moreover, ISIS likely doesn't even have enough money or resources to maintain control over the huge swath of land—roughly the size of Wyoming—that it has seized in western Iraq, according to Charles Ries, a former U.S. ambassador who's now a vice president and senior fellow at Rand Corp.

"I don't see that situation as sustainable from the economic standpoint," said Ries, who was the coordinator for economic transition in Iraq at the U.S. embassy in Baghdad in 2007 and 2008.

A fighter of the Islamic State of Iraq and the Levant (ISIL) holds an ISIL flag and a weapon on a street in the city of Mosul, June 23, 2014.

During normal times, the Iraqi central government provides the Sunni region of Iraq with about $500 million for salaries in the three provinces, and roughly another $500 million for investment projects and capital infrastructure, Ries said.

"That's a billion dollars a month, and it's hard to imagine that, while ISIS is quite entrepreneurial—they are doing kidnappings, and they are robbing banks, and they are charging taxes on trucks crossing their territory—but 500 (million) to a billion dollars a month is quite a large overhead.

Read More

The region controlled by ISIS has about 6 million people who are used to government largesse, Ries said.

"Iraqis are also accustomed to have subsidized food baskets that would be part of that cost, and I think it would be very hard on a continuing basis for ISIS to maintain the level of services that the people in the Sunni parts of the country have come to expect," he said.

'Like irrigating a field'

As for earlier reports that ISIS might be worth $1 billion, Ries concurred with U.S. intelligence sources who doubt the figure—despite the widely held assumption that wealthy Sunni Persian Gulf states are supporting ISIS and any oil that ISIS may be exporting from the parts of Syria it controls.

"I doubt they have that kind of income," he said. "And particularly $1.5 billion worth of liquid funds that they could use to support the Sunni areas of Iraq that they've taken over. But even if they do, it would only last a couple of months."

Ries said the inevitable shortfall that ISIS will face won't be obvious for a while, "but it's a little like irrigating a field. You have to continue to irrigate the field with money, otherwise the economy will dry up."

—By CNBC's Michelle Caruso-Cabrera.