So, how much will it cost to US to retrofit every international airport in the world where a flight from the US lands to have a completely seperate departure area only used for US-bound flights, and then how much will all of the expat work agreements with US government employees and contractors cost?

This is kind of like after 9/11 when Ma and Pa Kettle kept bleating about how there should be armed guards on all US flights. Most people grossly underestimate the number of airports and flights there are per day. I've asked many people this and most guess that there are "several thousand flights a day" with round numbers of less than 10,000. When I tell them it's closer to 30,000 just in the US (counting all of the puddle jumper flights) their eyes light up. You would need a workforce close to the size of the US Marine Corps to provide a couple of armed guards for every flight, and to put US customs at every foreign airport that has/might have a US-bound flight would be a truly enourmous undertaking.

It's done in Canada because, one, Canada is close and two, you don't have to spend a ton on BICE employees/contractors to live there. Also, a large portion of Canadian air traffic is US bound, so the economies of scale are there.

Just think about how much it would cost to build out and man a location that maybe has one US departure per day. Not just the pay of the BICE employees, but housing, car, medical, foreign taxes, per diem and/or monthly living allowances. Ex-pat agreements are expensive, even for the government. Just transporting household goods to and from a foreign posting can cost $10-15k easily.