I always undercut by 1 gil unless the person selling it has a value like "85391"

Then I just undercut by 100 because nobody has time to type all that.

Edit: That said, I do recall my boyfriend being angry at people selling something for 150-500k that took less than 25k to craft, so he undercut them all MASSIVELY by selling his stuff for 50k. That's a morality thing though, not really a greed thing. LOL