An easy way to get lots of money in this game is to use ZSNES' cheat option.

1. Start game and earn some money.
2. Go to Cheat ---> Search, and there you select how many digits you want to use, eg. 3 bytes mean up to 8 digits
3. Select decimal value so you can search for the amount of money you have.
4. Select exact value as you already know the value you are looking for in decimals.
5. Start search and then go back to the game. Go purchase some potion or something cheap and then go back to the search and enter the new value and search once more.

Continue to search until you only have one entry left because that's the one you have to alter. After you have altered it and gone back to the game you have to save a state right away, then go and turn off the cheat. Then all you do after this is load state and you got all the money you'll need and can always turn it on again if you need more. A good way to ensure you got money for the essential stuff like potions, tents and more. And not to mention that you need lots of GP if you plan on using Samurai job's skill Coin Toss... effective damage with GP coins.
Enjoy.