[oceanbreeze83]

This seems to take a very inclusive account of government spending but maybe not so for the attackers. We may need to consider the recruiting costs and ongoing operation cost for years leading up to the attack - even for expenses unrelated to the attack but part of the support and growth that led to it. You could argue that every AQ expense upto 911 was in some way building towards that event. I think if we have such a broad account of the government spend, we should do likewise for the other side. But your point is still good.

[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.