I don't know about the cost but the benefit is close to zero, if not zero. I am positive that the net benefit (because cost is surely positive) is negative accounting even for the number of jobs it creates.

> The source did not give an exact number, but said the failure rate was close to 80 percent. Members of Congress called the results "disturbing" in a public hearing following a private, classified briefing to the House Committee on Homeland Security.

https://www.usnews.com/news/national-news/articles/2017-11-0...

archived at https://web.archive.org/web/20221116171835/https://www.usnew... https://archive.ph/hzA1y

You can make one yourself: How many hijackings of US airlines where there in the 20 years before, and after. The number was higher before, but it was very small.

Locking the airplane cockpit door, along with awareness that hijackers will kill you (as opposed to fly you to a different country and you are free in a couple days if you stay quiet) are also factors. I don't know how to separate them from anything the TSA does, but I have to believe those are both more important.

> Locking the airplane cockpit door

A change which contributed to the complete loss of an aircraft and all aboard when the pilot left to go to the toilet and the copilot locked the door to crash it into a mountainside. The law of unintended consequences in full effect. Had such a change not been enacted that flight likely would have survived.

https://en.wikipedia.org/wiki/Germanwings_Flight_9525