I managed to farm up some 60M for Shirogane housing without even trying. The only real farming I did was a few hours of fishing for Aethersand (went for 100k each at the time), and a couple of Morphos, one of which I used for myself. I also had my retainers out on quick ventures for black/white dyes and other stuff that may or may not be valuable.

Old, dead content can also be lucrative, when it's the only source of an item. I remember things like music scrolls from Aquapolis going for 10M each simply because no-one was running it anymore.

Tl;dr: research which activities have the best gil per hour ratio, and abuse flavour of the month items!