Such analyses are often highly speculative, but this one seems to be based on concrete data:
And, contrary to speculation that Al Qaeda in Iraq was reliant on international donations, this wasn’t a source of funding either. The group was self-financing. In fact, the core organization of Al Qaeda in Iraq in Anbar province was so profitable that it sent revenue to associates in other provinces of Iraq, and perhaps even further afield. The group raised millions of dollars annually through activities such as simple theft and resale of valuable items such as cars, generators, and electrical cable, and hijacking truckloads of goods, such as clothing. And their internal financial record-keeping was diligent, with all the requirements of expense accounts in regular businesses. A central unit of Al Qaeda in Iraq’s hierarchy required operatives to keep records of even the smallest outlay and to turn over their “take” to upper-level leaders, who made the spending decisions.