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."