Hey, by accident I found a trick you can do on FFX.
Some of you may use this and some of you may think it's cheating.
In Besaid, teach Tidus Cheer and Haste.
Now, on the first boat to Kilika, when you fight Sin there will be a never ending line of 3 sinspawn. Rearrange your team to Tidus, Wakka, and Kimahri. Have Tidus cast haste on everyone and then keep using cheer until Wakka and Kimahri are doing at least 200, Kimahri should be able to do about 300 and Wakka should be doing about 230 for this to work nicely, now keep attacking the Sinspawn. They should die in one hit. When Sin sends out three more sinspawn, your team should all attack before the sinspawn. Now because you kill them in one hit they will never get a chance hit. Meaning you can attack forever. Hopefully if you cheered enough, at least one of your characters should be overkilling, mine was Tidus. Each Sinspawn gives about 20 gil and 2 AP I think. That's about one fight in Kilika.
Now heres the part you may not want to do, if you have turbo on your controller, turbo the X button [O for International] and hold it down with an elastic band or balance the edge of your Playstation on it. You can leave this for hours now, but keep checking just in case somethings gone wrong in battle or the thing holding the button down has come off.
I'm currently doing this well and it's working nicely!
Enjoy your money and levelling up.
P.S. If anyone has anymore tricks please share them!