FC searching can be daunting.

[Niwashi]

How much gil realistically do you need to start one?

The fee for registering the FC is 15,000 gil, and that's the only cost necessary, though there could be other more significant costs down the road at some point if you want to buy and decorate a house.

You could spend gil on other stuff in the meantime if you want to do things like offer gear to new members or run an FC event where you give out prizes or something, but otherwise the only non-house related cost is the initial 15K registration fee.

Housing is where the big expenses come into play. Before it's even an option though, you'll first have to get your new FC up to rank 6, so even if a plot is available and you're rolling in gil, you're not going to be able to get your FC house right away. Once you reach rank 6 you'd need to find an open housing plot. (I have no idea what plot availability is like on Adamantoise. Most smaller servers have plots regularly available, while on the crowded ones they're hard to come by.) You'll need both a plot and a construction permit to build a house. The plots range from around 1.5 to 3.75 million gil for a small one, 8 to 20 million for a medium, and 20 to 50 million for a large plot, and then the construction permit is another 450K, 1M, or 3M, depending on the plot's size. If you want a company workshop in your house, that costs another 800K, and if you want to decorate it with anything more than the random collection of whatever your FC-mates and yourself have accumulated from seasonal festivals and the like, there's no limit to what you can spend on furnishings.