"We aren’t going for the simple reason that we can’t afford it."
We aren't going for A (negate A) : A is not the reason. (We have another reason besides the simple reason)
We aren't going (negate going) : We are not going (We are not going because of the simple reason)
Literally, I think "We aren't going=We are not going=We don't go" and "for the simple reason that ~ = due to the fact that ~"
What's the meaning of the sentence?