Terrorists have to worry about logistics, just like everyone else. In the case of Islamic terrorists, there is the obligation to pay key personnel a living wage. This is so the poor fellow can take care of his family, since it is a religious obligation to have a family, and take care of it. While much is made of the suicide bombers themselves, who are often paid nothing, as they are on and off the job rather quickly. But your regular staff must be taken care of, otherwise you lose them. A successful terrorist organization has to keep its core staff. Otherwise, bad things happen. Chief among these unpleasant events are bombs that don't go off, or go off at the wrong time. The latter event will often kill key staff, and is bad for morale. It also amuses the enemy, whom you are trying to demoralize.
Currently, the largest Islamic terrorist organization is not al Qaeda, but the Taliban. Al Qaeda has several hundred people on the payroll to protect Osama bin Laden and a few key aids. Bits and pieces of intel indicate these fellows are somewhere in the mountains along the Pakistani/Afghan border. A few dozen more are elsewhere in Pakistan, where they have to be careful. Several bloody bombings, that killed many civilians, have made al Qaeda unpopular with many Pakistanis, and easier for the cops to track down outside the tribal areas along the border.
There are several hundred al Qaeda in Iraq, Syria, Jordan and the Persian Gulf states. Except for Syria, al Qaeda is on the run, and diligently hunted, in all these other states. No sense exposing any more paid staff than you have to. One of the main reasons al Qaeda came to Iraq in 2003 was that, all of a sudden, a lot of Sunni Arab Iraqis were cash rich, but security poor. With their man Saddam out of power, they were people without a country to plunder. Providing money and people to al Qaeda seemed a prudent investment in a more secure future. Didn't turn out that way, and a lot of the financing from the Saddam fan club has dried up. Same story in Saudi Arabia, where the most enthusiastic al Qaeda supporters and members decided, after Saddam was toppled in 2003, to go to war with the Saudi government. This did not work out well, and the government went after al Qaeda sympathizers and financial supporters as well as active members.
All of these attacks on al Qaeda "supplies" (of cash) has reduced the number of active members. Even those who are still actively serving the organization, are doing so at barely adequate wages. Income is down because of bad publicity (all those Moslems killed by al Qaeda bombs over the last three years), and lack of success (the Madrid and London attacks helped fund raising, but only for a while).
The Taliban, on the other hand, has been much more successful than al Qaeda. For one thing, the Taliban have remote areas where they are actually in control of things. That's why the Al Qaeda High Command still has a few people who have not been captured. But you have a hard time running things from a remote mountain village. What really makes the Taliban an attractive investment, for wealthy fans of Islamic terrorism, is their prospects of gaining control over even more territory in Afghanistan and Pakistan. Moreover, the Taliban have access to over a hundred thousand pro-Taliban Pushtun tribesmen on both sides of the border. Several thousand of these are currently on the payroll, to raise as much hell as they can before Autumn (when these gunmen go off the clock). Operating in this tribal environment has other advantages, as the tribes themselves provide something of a social safety net. Thus a wounded Taliban fighter has someplace to stay until he has recovered.
Finally, the Taliban also have access to money from the thriving drug trade in Afghanistan. Many drug lords "donate" to the Taliban not just out of belief, but as a form of insurance and a way to keep the police busy. But even though some wealthy backers are still loyal to al Qaeda and the Taliban (including some government money from Syria and Iran), the terrorists are increasingly turning to criminal activities to meet the payroll. These operations consist of run-of-the-mill criminal scams (credit card fraud, smuggling, theft) and some that have a touch of terrorism attached (extortion, and the demand for "revolutionary taxes.") The extortion angle is used in both Iraq and Afghanistan, against people who have money (businesses, usually), cannot walk away from their investments, and are vulnerable to terrorist attacks.
Alas, the growing dependence on criminal activities has been, for centuries, how violence loving idealists have been turned into common criminals. That's what happened to the Italian Mafia, and many similar organizations. It's happening to al Qaeda, turning them into gangsters who pray five times a day.