[SimbaOnSteroids]

Then do we count all the military R&D to develop the weapon systems used or the infrastructure for deploying a military response.

I think a better measure would be total funds that wouldn't have been spent otherwise (had 9/11 not occurred), as a measure. So for the US the costs to cleanup/rebuild Ground Zero and the Pentagon + the cost of deploying to Iraq and Afghanistan and whatever we're spending on Syria now plus whatever other operations happened as a result. For AQ it would be the cost of the attacks plus costs of expanding into Iraq and whatever involvement they had in Afghanistan.

This is to say nothing of the human costs in all this which we can all agree are the real tragedy.