Fool's cap is the most valuable herb for milling. It has a 50% chance to give Misty Pigment (instead of the usual 25% from other herbs) which converts in to Starlight Ink which is used to make darkmoon cards. A scribe can make one darkmoon card per day, and when you have eight cards, you combine them in to an epic trinket such as this one:
Notice that it is indeed binds on equipped! These trinkets sell for an easy 50'000 gold! Every dedicated raider will want to have one, so they will most likely sell just as easily as the ones in previous expansions.
I myself have three scribes producing cards everyday, and instead of buying herbs off of AH every day for thousands of gold, I went out with my gatherer to find a good spot to get Fool's Cap. This neat little spot gets me over 10 stacks in an hour. Thats over 2000g worth of herbs! I bet you are eager to go try this out yourself now, so heres the location
This is a screencap of my bags after about 50 minutes of farming here.
Keep in mind, this was done with a level 86 druid, without being capable of killing the mobs or using flying mount. When I hit 90 I can imagine this being twice as efficent, or possibly even bottable.
Here's a short video I made to show the spot and its potential. This was recorded on a medium populated server at peak hour, and there wasn't anyone but me there, showing how unknown this spot seems to be.