If we'd have left [the terrorists in Afghanistan] there and did absolutely not one damn thing to try and stop/kill them, well, how much did 9/11 cost the economy?
Quite a bit, actually. Took out a lot of infrastructure (including a major telecommunications hub and a number of business headquarters with all their personnel).
Then there was the cost of the reaction. For starters it stopped air traffic for days, and led to the creation of Homeland Security and all its costs - both direct and indirect (such as the large number of people who now drive rather than submit to the airport security theater.)
But I agree it was far less than the cost of the war that followed.
If someone walks in and shoots the party planning committee, how much does the next party cost the company? Trick question, there isn't one. Same with terrorists.
Actually, not the same with terrorists. Look up the term "blowback". Terrorists are hydras: Killing them tends to make martyrs, leading to the recruiting of more new terrorists than were killed
It also leads to diversification: The longer the tit-for-tat goes on, the less centralized and connected, the more independent and self-sufficient, the factions of the opposition become.
9/11 itself (along with his previous shot at the Twin Towers) was, according to Bin Laden, retaliation for the US bombing of a similar tower on his side of the world.