Final Fantasy X Tips

1,000,000s of Gil, Easily
To get huge amounts of gil in a short amount of time, simply buy four slotted armor from Kilika with HP +5% on it. Buy as much as you can fit into your inventory. (Sell off any "junk" armor you don't need.)

Go to the Monster Arena and overkill Kottos in the Monster Arena to get forty Healing Springs per overkill. (He sometimes drops 2 Dark Matters, but that's rare.) Kottos is an Area Creation that you get for capturing all the fiends on the Mi'ihen Highroad. Use those to customize SOS Regen onto the armor you bought and then you can sell it back for around 60 times or more what you paid for it. Just make sure you customize as many as possible after each two times fighting him, since your inventory can't hold more than 99 of the Healing Springs. Then, once you've customized them all, sell them off for huge gil. Go back and by more and repeat as much as you like. Works best if you've done some work customizing your characters and you can always hit Kottos for 99999 without taking too much damage from his counter.