> I understand why it costs this much. I don't understand why we think it is worth doing. Rather than fabricate a flying White House, why not deputize the vice president to carry out presidential duties while the president is in the air?

We have a national airborne command post, ultimately, in case the ground based command (the White House) is destroyed or under imminent threat requiring it to be (temporarily or permanently) abandoned. It's used for routine transport, and command and control while being used for routine transport, since it is desirable for it to be near the C-in-C at all times, and it doesn't make much sense not to use it.