Right, not so much an exploit and i dont suggest it if you need millions but for an extra few k its quite good.
Before you board the airship from Kazham back to jeuno u can speak to an NPC who gives you a quest to complete while traveling back to jeuno. This quest just involved finding the right person so is very easy and gets you 500gil ... once u get to jeuno take the ship straight back to kazham ( dont leave the port bit or ull have to pay again ) then when i kazham speak to the npc again and get straight back on the ship.
Its easy and for a bit of extra cash its ok, just dont try to make millions!