I have powerleveled (If you can call powerleveling a gathering skill that way, its not really that fast ;_;) herbalism 2 times on the same character, and I can tell you its definitely faster than mining, and probably (unless you have ALL the money at a time) faster than some tradeskills, because of the AH-mailbox running and all the time for crafting.
Depends on servers, but on mine peacebloom,silverleaf,earthroot, and even mageroyal sells for nothing, I prefer to grab them and give them as little presents to lowbie alchemists, and they are tremendously grateful, and its nice.
But once you get stranglekelp, briarthorn and up (excepting bruiseweed... it sells WORSE than peacebloom, again, depending on server) you will be rich.
With my 34 ally druid, I can just go herbing with walrus form (hehe, walrus) to the coasts of basically anywhere except the starting zones, and in 2 hours of herbing, i will have 3-4 stacks of stranglekelp, wich sells for about 7g a stack, not bad for a level 34.
And briarthorn... oh briarthorn. With my 70 lock I still pick that up if I see some of it on the way, it just sells so well and so fast its not funny.
Once you are 70 and have herbalism in Outlands, you will laugh at people that beg for money at high levels, so much that your lungs may implode.