Shadow: Primal shadow, on my server sells for about 12-15g, I farm on the Western edge of the world in Hellfire Peninsula with all the Voidwalkers, moderate drop rate, but easy to kill.
Air/Mana: I have a great farming location for farming Primal Air and Mana, (Note: This requires a Flying mount) In South Eastern Shadowmoon Valley there is a spot called the Altar of Shadow, there are Air elementals, so if you don't have a flyer farm here, but it can be heavily farmed, so it might be hard to get alot of kills. Anyway, fly a little north behind Dragonmaw Fortress there is Netherskates, which drop Primal Mana, and Air Elementals, also, if you don't have a flyer, go to Netherstorm to Wizard's Row, on a little island to the South West, lots of Mana mobs, but can be heavily farmed, also, Netherskates are incredibly easy to kill, but mana drain, so mana users should bring some water. On my server Air sells for 20-25g, and Mana sells for 18-22g.
Water: There are several locations, theres Slymer Lake in Terokker Forest, but they don't drop motes enough for me, I do the lake (forgot the name) in Nagrand south of Throne of the Elements, lots of mobs, easy to kill. Primal Water sells for 20-25g on my server.
Fire: I farm fire at Skald, in Blade's Edge Mountains, directly above Gruul's Lair, this is farmed alot, but not as much as Plateau of the Elements,easy mobs. Fire sells for 25-30g on my server.
Earth: I don't really farm earth since I'm a miner, but when I need to, I kill the earth elementals around Halaa and the Alliance town, I forgot name. Primal Earth sells for 12-18g on my server.
Life: Primal Life is usually not paid much attention as a Primal, but can make some good cash. I farm in North East Zangarmarsh at the Dead Bog (I forgot the real name, but I'm close.)
Enjoy