While Herbalism in general is very profitable, there are three herbs in particular that are extremely valuable.
They are needed as mats (materials) for pots (potions) that are in high demand from raiding parties.
That means there's a big market for them and heavy demand, which means lots of gold for you.
Frost Lotus generally appears as a drop among other herbs you harvest in Northrend.
That means you can't specifically target it (although there is one zone where it has its own nodes) and must instead find it by harvesting less-valuable herbs like Adder's Tongue.
Lichbloom is used in the Runic Mana Potion as well as some of the most important flasks, making it highly sought after.
While it can be found in most high-level areas of the game, nodes are the most prevalent in the Storm Peaks.
Your character should be in the high 70's before venturing here looking for Lichbloom.
Icethorn, like Lichbloom, is found only in high-level areas, with the highest concentrations in the Storm Peaks.
Aside from its use in Runic Mana Potions, Icethorn is used to make a few important flasks and elixirs, meaning you should find a ready market for any Icethorn you can get your hands on.
You don't ever have to worry about running out of WoW gold again.
The Warcraft Tycoon's Handbook is a top-quality gold guide that answers all your Herbalism questions and shows you exactly how your Herbalist can generate piles of gold.
Even better, the Handbook covers making gold in all sorts of ways, so you won't get bored doing the same thing all the time.
And everything in the Handbook is completely legal, so there's no chance at all of getting banned.