Most times I have flown spirit I got upgrades including bags dirt cheap. But even when I don't, they are still a pretty good price. As for southwest, it depend

Most times I have flown spirit I got upgrades including bags dirt cheap. But even when I don't, they are still a pretty good price. As for southwest, it depends on timing. They are cheap early on when major carriers are asking standard prices. Then as you get closer, southwest cheap seats fill up while major carriers start cutting prices, sometimes drastically if it turns out to be a bad travel season. Traveling with four bags is too much of a pain even when it's free to bring them.