It's worth mentioning now that arguably one of the best ways to gain money is in the Omega Ruins. If you have the Gillionaire ability equipped on someone (Rikku's Celestial Weapon has it when powered up), and you kill one of the Mimic boxes (they look like treasure chests but become a kind of evil birdman thing when you try to steal from them), you'll get 100,000 gil per battle. I personally prefer killing off One-Eye for money, usually once I start doing 99999 damage against him and can kill him in two rounds. His Triple AP stuff sells for a lot of money.
And don't be too disheartened by all of the key spheres. All in all it's not that difficult to find enough to clear every key node in the grid. What you will go nutty about is trying to customise the grid with +4 stat nodes![]()