There are many ways to get easy money. I can tell you two of the best ways that I know of. One of them is harder to get to, but it makes you money faster. Let me start with the easier, but less efficient way.
First, you must have access to the Feywood. I would suggest being at least level 32. Ok, well first, you will want to go to the Save Crystal, in Antiquity's End. Than, take a right at the fork, and you will find creatures called Mirror Knights. These Mirror Knights are not very powerful, and have 4000-6500 HP. They will sometimes drop this item called Windslicers Pinion. These sell for about 1,100 gil each! Mirror Knights are all over the area to the right path at the Save Crystal, and if you need to respawn them, go two areas way (I would suggest going the way you came, to Antiquity's End, and where you first entered Antiquity's End. ) If at all possible, avoid touching the Save Crystal, so that you may get a good battle chain. If you need some help understanding this, try using my youtube video. Link = http://www.youtube.com/watchv=CbwUcJHW8f0
The harder, but easier (loot wise) way is to farm Dark Lords. Now these guys are not hard themselves, but getting to them is the hard part. First of all, you must get the Stone of the Condemner, which can be obtained after defeating one of the Judges in Mt. Bur-Omisace. After defeating the Judge, talk to the Acolyte at the bottom of the stairs, just outside the Temple.
After you have obtained the Stone, go to the Stilshrine of Miriam. Once inside, go to the teleporter. The teleporter is guarded by two undead demons. After you defeat (or don't defeat) the demons, go to the teleporter. There are three options. Choose the one that asks to use the stone. After you teleport, make sure you are ready, because opening the door in front of you will trigger an Esper fight!
The Esper is Zeromus, and the fight is a tough one. Not only does Zeromus summon Dark Lords to help fight against you, but Magick is disabled in this fight! The only advice I can give you is, do not ignore the Dark Lords, as for they can become a nuisance. The other advice I can give you, is change all your healing gambits to use items instead of Magick. The last advice I can give you is, try to learn all the Phoenix Lore Licenses so that using a Phoenix Down will fully restore your character, and you will not have to waste time with Potions.
After you finally kill Zeromus, the Dark Lords will continue spawning in the room. Now you can stay here and chain the Dark Lords for some good loot and EXP. If the Dark Lords stop spawning, go use the teleporter three times and they should start spawning again. After you have accumulated the amount of loot you want, go and sell it for lots of Gil!