After furnishing my 4th house, owning all the minions and vanity gear that I like, and stockpiling materia/consumables, I would say the number you are looking for is around 200 mil.

If you can invest in buying a dead FC and bulding airships, those flying retainers can make you a small fortune once you are up and running with the latest parts. I can't even fathom how much gil I have made off of zu mounts.