For those interested in where the Islamic State (IS) gets its money, the Congressional Research Service (CRS) has produced a report detailing the financial pathways behind this entity.

As the report notes:

While IS funding streams remain fluid, the group's largest revenue sources appear (based on open-source information) to include oil sales, taxation and extortion, and the sale of looted antiquities. Oil sales initially provided the majority of the group's revenue, but gradually declined as a percentage of overall IS profits due to an extensive campaign of airstrikes by the United States and coalition partners against oil and gas facilities used by the group.

The money flowing into IS is substantial, ranging into the millions of dollars per day.  Consequently, senior U.S. officials view IS as one of the best-funded terrorist organizations – allowing it to administratively control and govern the territory it seizes.

Furthermore, IS has found sufficient internal financing to facilitate its operations.  This capacity to not require international funding – when coupled with its ability to effectively exploit seized areas for additional funding streams – puts IS finances "beyond the reach of some of the most common counterterrorist financing policy tools."

Despite the recent drop in oil price, IS revenues remain substantial:

Under Secretary of the Treasury for Terrorism and Financial Intelligence David Cohen in a November 2014 hearing reported that the Islamic State's revenue from oil sales had dropped from $1 million a day to several million dollars a week ... In February 2015, a Pentagon spokesperson stated that money from illicit oil sales was no longer the Islamic State’s primary source of revenue, but did not say what had replaced it.

IS oil is not able to trade on the world markets because of international sanctions, resulting in steep discounts of 80 percent or more when IS sells its oil at informal "markets" such as the Turkish border.

Another major source of revenue for IS is "the sale of antiquities looted from areas under the group's control," potentially rendering more than $100 million a year in funding.

The seizure of state-owned banks in several Iraqi provinces provided another $500 million or more in cash.  IS also levied a 5-percent tax on all customer cash withdrawals at private Iraqi banks.  Similarly, there are more than 20 banks in Syrian territory under IS control, but little is known about their financial arrangements.

Customs taxes and passage fees have been put in place by IS at border checkpoints.  Fees and bribes at crossings along the Iraq-Jordan border range from $200 to $1,000.

Taxation and extortion in IS-controlled areas could be yielding nearly $400 million a year.  For example, pharmacies in Mosul are being taxed at rates of 10 to 35 percent of the value of the drugs sold.  The provision of utility services also attracts a payment to IS.  Rates at some locations are in the neighborhood of $10 per month for electricity, water, and security.

Protection taxes and kidnapping are additional revenue sources.  Some Christian communities in IS-held areas have reported being threatened with death if they do not pay jizyah – often half an ounce of pure gold – or convert to Islam.  A United Nations estimate places ransom fees for IS kidnapping victims at $35-45 million in 2014.

The sale of agricultural goods such as wheat and barley could be generating hundreds of millions of dollars for IS each year.

While foreign donations do not dominate the IS funding stream, they are still significant.  During 2013 and 2014, up to $40 million may have flowed into the IS coffers from Saudi Arabia, Kuwait, and Qatar.

The challenge that Western forces are up against is illustrated by the following: "The Islamic State approved a $2 billion dollar budget for the year in early 2015, including a projected $250 million dollar surplus, designed to cover the costs of operations in both Iraq and Syria."  Consequently, as the CRS authors state, "U.S. efforts are centered on identifying new ways to target the group's finances by focusing both on the Islamic State and on others who conduct business with the group."