Money gained from monsters is completely random I think, albeit random and always low. The real way to make money is to fight enemies that drop items, and sell those. Or sell dungeon goods. When you start the game, you can really only fight goblins, and hornets. If you go to the woods northwest of Altea, or whatever the second town you see is called, there're a bunch of enemies that drop sellable items. Make your money that way, and you'll do fine. That's what I did.