In the past decade the average "value" of a hostage has increased from US$200,000 to around US$10 million - a bounty which supplements the propaganda and sense of terror generated by each kidnapping.
The documents reveal that governments which are willing to pay up do so in cash. The money is transferred via a series of proxies, and written off as development aid on national budget sheets.
But for Isis, kidnapping is just one arm of an extraordinary money-making machine which has helped make it entirely self-funding. The bedrock of this new economy of terror is oil.
Luay al-Khatteeb, a visiting fellow at the Energy Security Initiative run by the Brookings Doha Centre, has described Isis as "the world's wealthiest terrorist group". He argues that far from being a bunch of "nihilistic extremists", the organisation has created a sophisticated black-market economy over the territories it holds in Syria and Iraq.
Once it was dependent on foreign donors' money, with several hundred million dollars coming through social media fundraisers, private supporters in Kuwait, and - allegedly - Saudi Arabia. Now it is able to generate spectacular levels of cash as a result of its control of 60 per cent of Syria's oil assets and seven production facilities in Iraq. Oil is traded with a network of middlemen and criminal gangs, generating an estimated US$2 million a day. Some 30,000 barrels a day are sold on to neighbouring countries including Jordan, Kurdistan and Turkey.
It is also able to use the industrial assets and technical expertise it holds to produce refined products which it is able to sell for cash. Luckily for the rest of the world the majority of Iraq's oil resources are based either in Kurdish-held areas or in the south of the country which remains beyond its military grasp.
Alongside kidnapping, extortion and smuggling, it has been claimed that Isis scored a spectacular boost to its coffers when it staged what has been described as one of the biggest heists in criminal history after seizing control of Mosul. It was claimed gunmen made off with more than US$400 million at the region's central bank, although Iraqi officials have cast doubt on the story.
The group has also shown little concern at exploiting the region's extraordinary wealth of antiquities. One intelligence expert was recently quoted as claiming that it has raised US$36 million in 8000-year-old artefacts pilfered from the Qalamoun mountains west of Damascus.
- Independent