There may be spoilers ahead.
But while there's music and moonlight (you know the score)
ISIS much like a company release an annual report (how fucked is this) called the Al-Naba. This basically details kills, bombings carried out etc.
In 2013 ISIS carried out 615 vehicle bombings, including 78 suicide missions. Now the cost of this alone based on lower end calculations (C4 plastic explosives at $2,000 per kilo etc) would be $61.5 million (for real and that's just a starter).
Add into that the further 6,639 bombings the group laid claim to (improvised bombings, motorcycle devices you name it) and you have to think
How is this possible, where are the funds coming from.
Now the book states that ISIS make money from oil but it's also obvious that they are being funded and in a big way from other means but they are also incredibly resourceful and in a country riddled with corruption where they talk about millions of dollars as if its nothing, then its hardly surprising.
It was estimated 10 years ago that between 10 & 30% of the countries oil imports were being smuggled back out of the country and that's a 4 billion dollar top end. At that time they estimated half of that was going into the terrorists pockets.
Terrorism finance is a murky business, in June 2014 the CIA gained an incredible insight with the capture of information from an ISIS senior commander, detailing assets of around $875 million (Jesus wept is all I can say to that) and even worse this was before the capture of major city Mosul where it was reported they stole $429 million from the central bank. Pushing their net value to over 1.3 billion.
Now they are worth 2 Billion easily the wealthiest terror organisation in the world, second comes Hamas with half those assets.
When you think of whats available to them its absolutely frightening.