Why Elemental Earth farming is so lucrative
Elemental Earth is used primarily by Alchemists to make several kinds of potions. These Potions are much-needed for Raids, which is why a single hour of Elemental earth farming can up to 20-30 gold! On my Server, I regularly sell my farmed Elemental Earth for 2g, and it sells very well at that price.
Another great thing about this specific farm is, that the mobs with the highest drop chance for Elemental Earth are actually all around level 40. This means, that Elemental Earth farming is amazing to farm for your first mount at level 40. In addition to this, there is a bit of a gap in quests between level 38 and 42, so it is perfect to grind experience farming Elemental Earth for a while and getting the gold for your mount at the same time.
While there are more mobs here than in the Arathi Highlands, there are more players farming here. Additionally, there are a couple of quests at this location, which increases the competition some more. You should be aware that the Badlands is primarily a Horde zone, as the Alliance does not have a Flight Master here.
I recommend trying for the northern spot first, and if there are too many people there, check the ones in the south. Keep in mind though, that there are less mobs in the south.
Arathi Highlands Elemental Earth farming
Arathi Highlands has all the Elementals, and Earth and Fire are usually the ones that are farmed the most. Elemental Fire is still a lot more expensive than Earth is, though, so there should usually be fewer people at the earth spot. In the map below, I have marked the locations for all the Elements, just in case you want to switch over every once in a while.