Michael Freeman, "The Sources of Terrorist Financing: Theory and Typology," Studies in Conflict and Terrorism 34, no. 6 (2011): 461 – 47.(read less)
Although the following numbers are just estimates, they are indicative of the overall phenomenon: Al Qaeda’s annual budget was estimated to be $30 million; until the 1990s, the Provisional Irish Republican Army (IRA) had a budget of up to $15 million per year; at its peak in the 1990s, the Kurdish Workers Party (PKK) was thought to have an annual budget of $86 million; Hezbollah’s budget is between $100 million and $200 million per year, and may range as high as $400 million, the Fuerzas Armadas Revolucionarias de Colombia (Revolutionary Armed Forces of Colombia — FARC) in Colombia has an annual budget of somewhere between around $100 million and $1 billion; the Afghan Taliban raises somewhere between $240 million and $360 million per year; and all the different insurgent groups in Iraq collectively raised between $70 million and $200 million per year. As these figures demonstrate, the organizations that are responsible for terrorist attacks are much more expensive to operate than is indicated by the cost of any individual attack.
[Besides state sponsorship,] terrorist groups have increasingly sought funding through illegal activities. These include extortion or “revolutionary taxes,” kidnapping and ransom, theft, smuggling, petty crime, and pirating and counterfeiting goods…. Groups that are or were heavily engaged in kidnappings include the Abu Sayyaf Group in the Philippines, the FARC in Colombia, the Italian Red Brigade, the Tupamaros of Uruguay, and insurgent groups in Iraq, the Pakistani Taliban, among many others.
Many terrorist groups rely on the support of a sympathetic population or constituency as a source of funding. … Terrorists might also tap into sympathetic diaspora communities living overseas.… Many groups receive money in the form of charitable donations…. As examples, the Global Relief Foundation and the al-Wafa organization have been associated with Al Qaeda; the Holy Land Foundation for Relief and Development and the Quranic Literacy Institute have given money to Hamas.… Much of the money collected comes from Saudi Arabia and other Gulf states, where there are many wealthy donors sympathetic to those causes…. The Afghan Taliban also receive money from charities located in the Gulf countries estimated at between $150 million and $200 million per year. Charities are rarely entirely devoted to financing terrorist organizations; rather, they may distribute most of their money to other, legal causes, including other charities.