Factorio doesn't go on sale. The author's reasoning being it's disrespectful to those who paid full price for it.
Edit: official forum link stating thus: https://forums.factorio.com/viewtopic.php?p=159626#p159626
Edit: official forum link stating thus: https://forums.factorio.com/viewtopic.php?p=159626#p159626
I was wondering if I somehow missed it going on sale, or it never had.
Glad I know it isn't likely to any time soon.