Before we get started if you are still leveling your inscription, the Forged Documents are a fantastic (though slow) leveling mechanism. Planning around this can save you a lot of gold. Also you do not need to be level 84 to reach 525 inscription.
Here are tips on making those painful and wonderful Darkmoon cards. Note that these tips are for players making several dozen cards for multiple decks. If you are trying to make a particular card my suggestion would be prayer.
1) Herbs: It will take between 24-40 millings of herbs to make a single card. That is 6-8 stacks of herbs to make a card. Which herbs you want to use is based on the AH cost. I milled 1000 of each herb to see the yielded pigments. It turns out that there are only two tiers. The lower tier contains Azshara's Veil, Cinderbloom, Heartblossom, and Stormvine. The upper tier contains Whiptail and Twilight Jasmine.
Each lower tier millings (1000) yielded approximately 50 inferno inks.
Each upper tier millings (1000) yielded approximately 83.35 inferno inks. This means the upper tier is 1.62 times the value of the lower tier.
If lower tier herbs are 100g/stack then cost threshold is 162g/stack for upper tier herbs. Let the cost make the herb decision for you.
The only other consideration is how long the milling will take you. So if you are milling then you are not off questing, etc. and milling the lower tier herbs will take longer.
2) Card Trading: Even though all four decks are very good, it is doubtful they have the same value. There has not been enough data to say which decks are worth more. But we do know that the two of stones is just as likely to be created as the eight of stones. So trading cards within a deck should not be a concern.
I personally view all four decks as having comparable values and have been trading cards based only on what I need. As prices have stabilized a bit, it is very clear that
ALL DECKS ARE NOT OF EQUAL VALUE. In particular the stones deck appears to be the least in demand. I have had trouble selling them despite a very low cost.
3) Put an alt in Dalaran to make Blackfallow to Inferno trading easier.
4) Milling Macro: This macro is essential.
/cast milling
/use Azshara's Veil
/use Whiptail
/use Stormvine
/use Twilight Jasmine
/use Heartblossom
/use Cinderbloom
5) My Data: You may want to note that my numbers are different than those presented on WoWhead. They present you with a range of pigments from the milling. However the distribution within that range is not uniform. Based on WoWhead data the tier yields are 73 and 128 which is well above what I was able to produce.
I will try to clean this up with better formatting and I am sure there are several items I am missing.