One of the most successful ways to go after an Islamic terrorist organization is to figure out how they are being financed and attack their sources of income. Since 2014 ISIL (Islamic State in Iraq and the Levant) has thrived via looting and smuggling oil and antiquities (which Syria and Iraq still have plenty of in the ground). ISIL considered the freshly excavated antiquities as valuable minerals and organized a regulation system for the many existing groups and individuals who illegally searched for and sold these antiquities. ISIL legitimized this and set up official ISIL markets to buy the antiquities and then arranged for them to be smuggled into Turkey were existing criminal gangs had, for generations, been manufacturing false documentation for these items and getting them into legitimate markets. For decades Turkey has been under pressure to shut down these gangs but not a lot was done. Apparently the gangs paid well so they could continue to operate. Some of the ISIL officials had started out working for Saddam Hussein in the 1990s, smuggling out antiquities when Iraq was under UN embargo until it paid Kuwait for damage done during the 1990s occupation. In 2003 these guys were out of business but many went to work for al Qaeda in Iraq and later ISIL (which is what al Qaeda in Iraq evolved into). The U.S. has been tracking some of these Saddam era antiquities smugglers since the 1990s and that helped to track down the ISIL antiquities smuggling bureaucracy and, in a series of raids, seize some of the key operators and their records. That led to a lot of damage being done to the antiquities smuggling network, including the people and institutions in the West and elsewhere who had ultimately paid high prices for these recently (and illegally) excavated antiquities. It turns out that ISIL was earning over $5 million a month from this business before that cut by a third or more because of the raids and arrests. Similar damage was done to the illegal oil exports by bombing the wells and tanker trucks.
Cutting off ISIL from much of its financing has done real damage to ISIL combat capabilities. Desertions are way up since late 2015, with many of the deserters reporting they left mainly because they weren’t being paid. ISIL also can’t buy needed supplies. Not just weapons and ammo but also food, medical supplies, vehicles and bomb making materials. Cash is also needed for the occasional bribe. The fact is that every armed resistance on the planet can trace its origins, and continued existence, to someone who knows how to get the needed cash, and spend it on needed items. Even when your fighters are not being paid you need several hundred dollars a year per man for food, ammo and other supplies. The most common money making activity is large scale extortion. They call it levying "revolutionary taxes" on those that can afford to pay. Many rebel groups find teenagers an excellent source of manpower. The kids are often kidnapped and then convinced, brainwashed or coerced to join the rebel forces. But the kids had to eat. Weapons and ammo had to be bought, or stolen from the police or army. Most new recruits would initially serve as porters, and laborers around the camp, while getting some training on how to handle a rifle. When enough weapons were obtained, or enough existing fighters got killed, or deserted, the new guy could be armed. Sometimes, the new recruits would go on raids without a weapon, simply because there were none available for him. During the fight, the unarmed recruit would be expected to grab a weapon from a fallen comrade, or collect one from a dead opponent after the battle was over. To put it simply, most rebel movements are low budget operations. But they do have a budget, and to survive, they must work within it. Islamic terrorists excel in one area and that is the degree to which they tend to establish a fund raising operation and a well-documented bureaucracy to spend the money raised.
ISIL, Al Qaeda and the enemy fighters in Syria, Iraq and Afghanistan are no different. Much data on current pay rates is obtained from interrogations of captured fighters in Iraq and Afghanistan. Captured documents also confirm that, while the fighters believe, to varying degrees, in why they fight, they generally won't do it for free. Suicide bombers are an exception, although sometimes they are encouraged to volunteer by the promise of a large (a few hundred, to tens of thousands of dollars) payment to their families. Otherwise, the pay varies with how dangerous the job is in Iraq. Most of the violence is carried out by Sunni Arabs who used to do Saddam's dirty work. They were paid then, and since they got tossed off that payroll in the 2003, they expect to be paid to do the same kind of work (terrorizing people). The Taliban and al Qaeda come a little cheaper. In Afghanistan, Taliban fighters serve for the "campaigning season" (roughly April to September) for anywhere from $150 for $300 a month. Commanders get more. When al Qaeda was based in Afghanistan, they had a fairly elaborate payroll structure. That has largely disappeared, but it's still understood that full time al Qaeda operatives get paid regularly, and larger expenses (like travel, or weapons purchases) are covered. This money is still being raised from via Islamic charities and wealthy Arabs who believe in al Qaeda (a world government run by Islamic clerics, who will be grateful to merchants and businessmen who supported them back in the beginning.)