A friend of mine watches BBC Top Gear.
He sometimes hears the guys say "That would be that". What does that mean?
I don't know the context, but generally, this and similar phrases (e.g. "that's that") are generally used to mean, roughly, "The issue/task/event/discussion is done." It is generally used somewhat ironically, in cases where it has become obvious that the situation is resolved.
For example, imagine you are debating with someone about the airspeed velocity of an unladen swallow. A third person walks up, opens their phone, and finds the answer online. The discussion is complete, since the answer is found, and everyone knows it. An appropriate response would be, "Well then, that would be that."
This is a silly example, but I hope you get my meaning. It is difficult to explain without hearing it in context.
Another use can be seen here:
"I realized Otto would probably just shake my hand formally and we would have very civilized time and that would be that."
The explanation there is given as: "That would be all (there was). There would be nothing more."
Here is a snippet from King's 'The Stand'
Nick was depressed and slightly overawed by the size of the country; never before had he realized how easy it was to stick out your thumb, knowing that sooner or later the law of averages was going to favor you. A car was going to stop, usually with a man driving, and with a can of beer resting comfortably in the fork of his crotch more often than not. He would want to know how far you were going and you would hand him a slip of paper which you’d kept handy in your breast pocket, a slip of paper which read: “Hello, my name is Nick Andros. I’m a deaf-mute. Sorry about that. I’m going to _______. Thanks very much for the ride. I can read lips.” And that would be that. Unless the guy had a thing about deaf-mutes (and some people did, although they were a minority), you hopped in and the car took you where you wanted to go, or a good piece in that direction.
I think, in this context it means something like 'And it was a usual way things happened' or 'It used to be like that'