In reply to tony:
> I doubt much of the locally-derived revenue from extortion is electronic.
Yes, but that can only last for a short time, essentially a slash-and-burn system where you pillage any existing wealth in an area but do nothing to increase it, while destroying any productive capacity in the area you control. Much as looting conquerors have always done, despoil somewhere and then move on when you have destroyed that place, which is partly why they use brutal conquerors methods of terror and intimidation, like Ghengis Khan or Tamurlaine. So they need more long-term and reliable sources of revenue and those can be gradually tightened up on.
Some of course is reported to come from looting antiquities, but that is probably small, as they are difficult to transport and dispose of.
It is dangerous to imagine that Islamic extremists like ISIS are stupid, underestimating your enemy. They are certainly deeply malign and barbaric, but there is frequently a logic to their actions, perverse though that logic is to us. And that logic requires continuing supplies of resources, including readily usable money to keep going - they have to maintain the "Caliphate" to keep up their Islamic USP, despite claims that if it were removed, they would simply move elsewhere, they have stuck their flag to a particular, though variable, piece of territory, it would be a considerable loss of face for them to be removed from it. So trying to dry up their finances certainly IS worth doing, even if pretty difficult to do comprehensively.