On my server, prices on the AH are:
Fjarnskaggl: 85
Foxflower: 85
Starlight Rose: 175
Flask of the Seventh Demon: 2000
Each Flask of the Seventh Demon's crafting cost is around 2925 considering AH prices. (7x Starlight, 10x Fjarn, 10x Foxflower)
So for each try that you make a flask of the seventh demon, you are losing approximately 1000 gold. (if you're chasing rank 3)
Now if you have rank 3 of this flask, you have the chance to get extra flasks.
From the comments I've read on Wowhead and other forums, each craft results in about 1.5 flasks. This is taking into consideration that some procs give you x20 and some give you x1.
If the cost of this flask is only 2000 on the AH, and the expected return is 1.5 flasks per craft, that means you're getting about 3000 gold (2000 x 1.5) for 2925 in mats.
As more and more people get rank 3, I can only expect the prices of the flask itself to go down on the AH.
So if you're one of the unlucky ones like me who have yet to get rank 3, it makes better economic sense to sell your herbs than hope RNJesus blesses you.
Everything above depends on your server's herb/flask prices, and if Blizzard buffs/nerfs the proc rate.