Then who pumping billions of dollars on ISIS ? Bhutaan ?
It would be amazed for you to know how a Legitimate funding can be used to fund terrorist organisation...
Like a buying groceries in supermarket, or buying books in the book store, or getting a coffee in your local shop, if you are doing that, YOU MAY be sponsoring terrorist yourself.
The latest report suggested ISIS made 12 million per month alone in Mosul during 2014. And that is only one city. The combination of legal and illegal funding included
-Water/Electricity control.
-Oil procurement
-Donation
-Extortion, Kidnapping, Money Laundry.
-Legal Trade
-Human trafficking
-Drug/Opium trafficking
How ISIS operate is much like how Drug Lord operate in Mexico, Columbia and in the Street of Brazil. Minus the oil.
The problem is, most of the way ISIS Funded is underground, and they don't pay tax, which mean what they earn is what they get, and the full amount they can use to fund their war. In Mosul alone, there were about 140-200 high profile kidnapping of wealthy individual, that alone would have bring in tens or hundreds of millions of dollars.
Another way is to extort criminal organisation, in common practices, it was called "Creaming Tax" which mean they charge a % of money on top of your earning to allow Illegal activities such as Gambling, Prostitution or any other illegal activities to go on their territories, That way, you don't get the competition and you don't have any cost associated with this operation.
Human and Drug trafficking is a by-product of planning terrorist attack all over the world. If you can smuggle Terrorist overseas, you can smuggle women as prostitute to South East Asia or UAE or Drug in China or Russia. Which have a high dependence base.
Oil - Do I need to say more?
Donation is money coming from legitimate source when they were either diverted or laundered to appear into their account, of course you cannot curt a check to ISIS, but you can cut a check to a local group, charity or community group, they could have been used, in part to fund a legitimate cost, but it could also used to funnel money that ISIS received illegally.
Water, Electricity, infrastructure control mean they would charge a high price for these infrastructure to offer to local, they don't paid the worker and they have more than abundant source to generate electricity (well, they are in Iraq for crying out loud) in fact, ISIS may had supplied electricity and/or water to surrounding country, like Turkey, Iran, Syria.
Legal Trade is basically anything that legal, like tailoring, retailing, home building, or anything that you would do in normal day to day life. There were rumoured one of the big fashion brand had their textile came from ISIS controlled Iraq and they were smuggled into Turkey and ship it as product of Turkey to by pass sanction.
There are sanction, and there are always way to by-pass this, it's quite easy to funnel money into a group if you are outlaw controlling the land, and you have a lot of people.