For this farm we're taking it all the way back to eastern kingdoms. We're going to be killing the purespring elementals in the purespring cavern for the item essence of water. Essence of water is a semi-rare drop that goes for a shocking 252g per piece. The caveat is that it doesn't drop all too often, but is still worth the trouble. I wasn't getting TOO many of these, but I was still making bank with them. The essences are used to create the old-world illusions, so they do have some inherent value. Simply patrol the area and kill as many elementals as your heart desires. It's a really cool and fun farm that can be done on autopilot. Usually there is noone patrolling the spot.