If you're in need of some Gil and have access to the eleventh chapter, try the following. When you reach the section of the eleventh chapter where you're ordered to kill all of the Deep Ground Soldiers on the central pillar, kill all of the enemies as instructed. Then, stand on one of the circular pathways nearby the pillar. Small robotic enemies will come out of the various holes in the wall. Each will drop 500 Gil when defeated (and 30 experience points as well). With some patience, you can make a lot of cash.