A great way to get as much money as you want is by attempting the following. Once you acquire the spell called Quake, head to the Master Tonberry quest. Use Quake in every battle, which should deal the maximum 9,999 HP of damage while using 58 MP. An Ether costs 200 Gil, but a dead Tonberry is worth 500 Gil. You should therefore make a profit of 700-1,200 HP per battle. Repeat this process as much as you can to theoretically make as much money as you want.

Addition from dinamo_badblueboys:

Additionally, you can just kill the Tonberries on mission 10-2-3 and not have to use any MP, therefore saving any spent money. They only have 8,000-10,000 HP, and are easy to kill without Quake by the time you acquire that spell.