I don't know what to farmFollow

Try going to La Theine Plateu on windy days and in the small "hole"/indention looking things you'll see Air Elementals. Kill them and they are guaranteed to drop at least one Wind Cluster which currently is selling for 3k in most places. I've seen them drop up to 3 clusters but have never tried hunting them with Treasure Hunter which I am sure will more than double your profit. However I must offer this warning Air Elementals are level 18-20 yet they have massively high defense. They will cast aero on you often and hit you frequently. At your level you should be fine to kill one or two and then rest but do not try mass farming them because you may be in for a surprise....

p.s. just a little comment, mnk isn't nearly as expensive as some other jobs out there...

If you go to Davoi, the orcs in the first room upon entering drop a little bit of gil and lizard armor. There are also a lot of lizards in Davoi if you like to farm those.

Yagudo in Giddeus are also good for farming. They drop gil, the BRD-type drop scrolls, there are silver beastcoins, and you can synth the yag necklaces: wind crystal + Yagudo necklace = 3 spools grass thread, 3 spools grass thread + earth crystal = 1 grass cloth. Also, lightning crystal + Yagudo necklace = 1 copper ingot.

Buburimu peninsula is also a nice overlooked farming place. Bull Dhalmels drop dhalmel meat, dhalmel hides, and giant femurs. Usually when I farmed there, I would just run a loop around the area and kill as many dhalmels as I could til inventory was full (just hold onto the cup of saliva you get, if you keep dropping it you'll just keep getting it) and then go into Mhaura and sell the stuff to vendor. Meat ~60g each (almost always drops), hides and femurs ~200g each. This goes for Tahrongi Canyon as well, there are Dhalmels, Yagudo, and Crawlers (drop silk thread) there.

Passhow Marshlands are great for farming also, there are water and lightning elementals that spawn there (be careful because elementals are extremely strong against physical attacks). Also malboros, goobbues, quadavs, and goblins can have some nice drops.

Off of Passhow is Beadeaux, though I'm not sure what lvl the mobs are there. But hey, if you're ever farming Pash and want to pop in to con stuff, probably a good idea. Quadavs drop gil and brass armor, and old quadavs drop quadav helms, which are non-stackable, but go for a repeatable quest in Bastok at 1400g each.

Ghelsba is a lot like Giddeus, just train and kill mass amounts of orcs for gil. Same goes for Palborough Mines, there are other random item/armor drops that can be sold.

Other tips for money making:
1. Take up mining. Pickaxes can be bought in Galvin's Gear in Bastok, and run around Palborough Mines looking for mining points. Ore can be sold for a good amount of gil oftentimes, though it takes a lot of patience to mine.
2. Logging. I've never personally logged, so you might want to ask someone that has about where some nice spots to start out logging are. :)
3. Harvesting. Sickles can be bought at Ensasa's Catalysts shop in Windurst Waters, and harvesting points can be found in Giddeus, around Sarutabaruta, in Yuhtunga Jungle, and Yhoator Jungle.

Good luck to you and hope this helps a bit!^^