A UN report on sanctions against the Taleban and al-Qaeda - due out formally on Monday but seen by the BBC - claims that sanctions and other measures taken by the UN have so far "achieved less than was hoped" and have had only "limited impact".
Attacks can be cheap, says the UN
Immediately after the 11 September attacks, the money trail attracted a lot of attention as the US tried to gain global co-operation to choke supplies of cash that were thought to be going to al-Qaeda.
The UN report confirms what many involved in counter-terrorism have increasingly come to believe - that, whilst they have their uses, sanctions and other attempts to stem the flow of money into al-Qaeda are not necessarily the most effective way of preventing future attacks.
For instance, no nation has reported blocking an arms sale or preventing travel to anyone on the UN list.
Recent reports - both from the UN, and the US independent commission into the 9/11 attacks - have emphasised a number of key facts about al-Qaeda's operations and its evolution which have altered our perspective on the role of money within al-Qaeda.
COST OF ATTACKS
Madrid bombings - less than $10,000
Bali nightclub bombings - less than $50,000
US embassy bombings in Kenya and Tanzania - less than $50,000
Attacks in Istanbul, Turkey - less than $40,000
9/11 attacks - $400-500,000
Firstly, Al Qaeda's terrorism is relatively cheap. The UN report estimates the Madrid attacks - which killed 191 people in March 2004- only cost about $10,000 (£5,600).
The Bali nightclub bombings in October 2002 killed 202 people and cost less than $50,000 (£28,000), as did the 1998 embassy bombings in Kenya and Tanzania.
Only the 11 September 2001 attacks cost more - the 9/11 commission estimated that the plotters eventually spent between $400-500,000 (£223-280,000) to plan and conduct their attack.
The relatively low cost of most attacks is partly because al-Qaeda and its affiliates have used relatively low-tech means to carry out their attacks. In Madrid, perpetrators used stolen mining explosives and mobile phones rigged to act as detonators.
Secondly, al-Qaeda has proved highly flexible and adaptable.
Bin Laden has not been sighted since 2001
This has partly been forced onto the organisation because of the huge disruption generated by the US and broader international community, ranging from the invasion of Afghanistan through to pressure on countries in the Gulf to prevent money flows.
The UN report makes clear that the transformation of al-Qaeda into a "loose network of affiliated underground groups" which can operate largely independently against local targets of their own choosing, using limited resources, actually makes central flows of money less relevant.
So the failure of sanctions is partly because "they address a set of circumstances that no longer apply" and therefore need to be adapted and updated.
One step ahead
UN sanctions require a travel ban and arms embargo against individuals and groups linked to the Taleban or al-Qaeda- currently 317 individuals and 112 groups. Sanctions were first imposed in 1999.
The problem is that al-Qaeda has responded by adapting to find ways to move money.
UN sanctions have been largely reactive whilst al-Qaeda has a proven ability to adapt, be flexible and stay one step ahead of the authorities.
Large flows are also less important because al-Qaeda cells and affiliates are often self-financing.
The 11 September hijackers - who ran the most complex al-Qaeda operation - did receive cash from abroad.
But in other cases - like Madrid and even the African embassy bombings - cells were forced or encouraged to rely on crime to finance themselves, sometimes petty crime like credit card fraud.
That has become ever more the case since the new restrictions put in place post 9/11. In some recent cases, terrorists in Europe are reported to have operated immigrant smuggling and passport forging rings.
'Fertile fund-raising ground'
The 9/11 commission also made clear that vast sums of money were less important to al-Qaeda than had sometimes been assumed.
It undermined the notion that Osama Bin Laden himself had a $300m (£167m) fund which he used to bankroll the organisation.
In fact, he received about $1m (£560,000) a year until 1994 and in the mid-1990s there are actually accounts of disputes between Bin Laden's affiliates about why some people were getting paid less than others.
Pre-9/11, it is estimated by the CIA that it cost about $30m (£17m) a year to sustain al-Qaeda's activities and most of this came from donations from wealthy individuals and through charities, especially Saudi Arabia which the 9/11 report describes as "fertile fund-raising ground".
But much of the money was spent on keeping facilities going in Afghanistan and also in paying about $10m-20m (£6-11m) a year to the Taleban for safe haven. Now of course, all those costs are gone and the core of al-Qaeda needs far less to operate.
Because of all these changes, the US is increasingly using the money trail as a way of tracking al-Qaeda and trying to uncover the complicated links that make up the network, rather than moving straight to freezing assets and trying to disrupt the flow.
In many cases, the US is now saying that it is easier to watch, observe and investigate rather than simply cut off the flow of money. This is because only through the process of investigation that officials can uncover new cells and understand the operations of al-Qaeda as it disperses into an increasingly decentralised network.