Helen Lock ('How Isis became the wealthiest terror group in history', Independent 15 September 2014) joins those alarmists stressing how wealthy ISIL has become:
Islamic State (Isis) militants are taking in more than $3m (£1.8m) a day through oil smuggling, extortion, theft and human trafficking, say intelligence officials and experts. Aid workers, researchers and intelligence agents say that as the group has taken over sections of Iraq and Syria it now controls as many as 11 oil fields in both countries. It is using generations-old smuggling networks to bring in the trade from oil and other goods. It is also making money from extorting cash from people in the areas they have seized, kidnapping for ransom and from human trafficking.While stressing how much money can be made from smuggling oil (quoting Denise Natali, a senior research fellow at National Defense University and Luay al-Khatteeb, a visiting fellow at the Brookings Institution's Doha Center in Qatar), other sources of revenue are sought "Its cash-raising activities resemble those of a mafia-like organization," another US intelligence official said. "They are well-organized, systematic and enforced through intimidation and violence".
As well as selling oil the group has earned hundreds of millions of dollars from smuggling antiquities out of Iraq to be sold in Turkey, al-Khatteeb said, and they are making more from human trafficking by selling women and children as sex slaves.That figure of "hundreds of millions of dollars" seems more due to popular conceptions of "how much" archaeological treasures are worth in financial terms ("priceless") rather than any real hard figures. Although we can be pretty sure that antiquities smuggling is producing profits for ISIL, I doubt very much that it is in the same range as revenue as the oil and other sources.