Very cool question. We don't have the answers, but here are a few thoughts.
Who benefits? There is only one beneficiary of Al Qaeda as a virtual actor: the US Military-Industrial-Intelligence-Congressional Complex whose outrageously wasteful funding and excessive (70%) obligations to contractors are bankrupting the US economy, but who cares as long as the corporate gravy train keeps rolling along. The indigenous peoples seeking self-determination, including the long-repressed people of Saudi Arabia and the long-repressed peoples of Palestine, do not benefit from a model that Mahatma Gandhi clearly understood was self-destructive. Non-violence is the only sustainable path to self-determination.
Calculating value. With the above firmly in mind, Al Qaeda's “value” to the sole beneficiary, the MIICC, is a combination of three sums:
1. The sums Al Qaeda and related groups receive from governments, corporations, and individuals interested in sustaining radical Islamic violence against both Muslims and the West.
2. The sums the US and others spend on false flag operations attributed to Al Qaeda (the underpants bomber is probably an Israeli false-flag operation with US consent and colalboration)
3. The sums the MIICC receives from a corrupt Congress that has not done a serious national security baseline evaluation of need since Senator Sam Nunn (D-GA) retired from his post as Chairman of the Senate Armed Services Committee.
What is Al Gaeda Worth? Our wild-ass but informed guess is $1 trillion a year. That one trillion a year is both positive value (for those that benefit) and negative value (for all the others). With one trillion a year we could have brought the USA into the 21st Century, funded free cell phones for the five billion poor so they could create infinite stabilizing and self-sustaining wealth, and created a prosperous world at peace.
Afterthought: Now add that to the $2 trillion a year in crime, one half of which goes to pay bribes to dishonest government and corporate officials who help transnational crime, and then add the 50% in waste that has been documented for the US health care industry by PriceWaterhouseCoopers, with a similar waste factor assumed for the rest of the federal government and especially the US military that is optimized for 10% of the threat (inter-state conflict) and you get an idea of just how totally hosed the national decision-making process is–we are so out of touch with reality, including true cost and true capaiblities information, that we are about to spend $12 billion a year on cyber-security, when we have determined there are only 63 individuals in the USA actually qualified to work at the code level in this domain.