Later on on the game shit gets so expensive that the Llusu Mine/Bone Fragment technique will only by you 5 or 6 pieces of equipment.

However if you go to the Henne Mines, head to that third gate switch that you need to hit. When you flip it, flans will drop from the ceiling. Hit the switch 4 more times so that they keep dropping, then chain away. They'll drop yellow liquid that sells for at least 400 gil.
when that's done, clear 2 screens, then return to the spot and do it all over again. by the fourth or fifth time, if you don't break the chain by attacking bats inbetween, you'll have 99 ellow liquids, and over 40,000 gil