Do we put a comma before "then" when it's the last word in a sentence?

Do we put a comma before "then" when it's the last word in a sentence?

If she goes, I'm not going, then.

Solution 1:

There are examples where you should use it, and others where you shouldn't.

Is she going? I'm not going, then.

In this case, "I'm not going, then" is equivalent to "Then I'm not going". If you change this example to a single conditional statement ("If she's going, then I'm not going"), then — as the comments indicate — you would not transpose 'then' to put it at the end of the sentence.

Q. Shall we meet at eight o'clock?
A. Sure, let's meet then.

In this case, "then" is a reference to the phrase "at eight o'clock" used by the person asking the question.