Rarely Found Herbs
Some herbs, such as Wildvine, Grave Moss and Ghost Mushrooms, can only be found occasionally while gathering other herbs or are located in out of the way areas such as graveyards or caves. Because of this, they are quite rare and difficult to find on the auction house. These herbs are used as crafting reagents for low-level equipment and enchants; players who want to obtain the best possible equipment and enchants for their low-level character would be willing to purchase the herbs at a high price.
Rarely Gathered Herbs
Certain herbs may command a higher price on the auction house simply because only a few players gather them. For example, herbs such as Nightmare Vine, Mana Thistle and Netherbloom are gathered from areas players visit only rarely, and as such supply is scarce; however, players who are trying to increase their skill in the alchemy profession or trying to craft a specific item may still require large quantities of them.
High-Level Herbs
Herbs added to the game in the most recent expansion usually command a high price, especially when the expansion has just been released and only a handful of players have a high enough herbalism skill to gather them. Because these herbs are a necessary reagent to craft the most powerful stat-enhancing potions and elixirs, a large amount of players will be interested in purchasing them.
Considerations
The value of herbs constantly changes depending on the current supply and demand. High-level herbs may not command the highest price but will most likely make up the majority of your earnings due to the fact you can sell them in bulk, while rarely found or gathered herbs will sell for a higher price but more infrequently. Consider creating a character dedicated to the storage of herbs; equip him with a set of herb bags, such as Hyjal Expedition or Emerald bags, and use him to store at least one stack of each herb type available in the game. This way, you will be able to react more swiftly to temporary demand surges for a specific type of herb, increasing your profits.