As you probably know, discoveries are the best exp return per mats used, and Jewelcrafting is the cheapest crafting discipline for one good reason: You only need 1-2 gems for a discovery, and if you use filligrees (?) you can actually get 2 discoveries out of 1 gem.
On top of that, you can transmute low level gems into higher level ones which may be cost effective should a certain tier be inflated.
Armorsmithing/Huntsman/weaponsmithing require tons of blue quality crafting materials (3 per discovery for first tier, and then 8 for second tier, and 15 for rares). The costs vary quite a bit but ultimately most of the time the total cost per discovery ends up costing more than a single gem, so you may end up spending as little as 1/8th the amount of money for a discovery.
So IMO Jewelcrafting is the easiest and cheapest exp for leveling. The only downside is that gold tends to run pretty expensive so that tier is a bit painful. The stuff is easy to find in Dredgehaunt Cliffs though.
Cooking can be good for leveling because of the sheer amount of discoveries it has but the complexity (having to look up stuff) and inventory space it requires makes it not very practical for leveling IMO - not ot mention that some ingredients can only be gotten from heart vendors.