I haven't been working Inscription for very long. I started two or three months before the end of Wrath and wasn't even very serious about it at first. All I really wanted was a secure supply of cheap velum for my enchanter (naturally a few days later the word came out that velum would be sold by a vendor in the next patch). Since then I've gotten a lot better at Inscription. Recently I've come to the conclusion that it doesn't matter very much what the current prices of herbs are. I can follow the market through it's ups and downs buying herbs in complete confidence I won't lose gold by using simple math and rules of thumb.
There are basically 2 tiers of Cataclysm herbs. First tier herbs, including Cinderbloom, Stormvine, Azshara's Veil and Heartblossom, give on average half an Inferno Ink and 5 Blackfallow ink per stack. Second tier herbs, Whiptail and Twilight Jasmine, give on average 1 Inferno Ink and 6 Blackfallow Ink. 10 Blackfallow Ink can always be traded in for 1 Inferno Ink meaning we can consider Blackfallow to be worth at least 1/10 of what Inferno goes for (note this will always be true though the opposite will not always be true).
To keep the math simple I consider all glyphs to cost 10g each to craft and Blackfallow Ink to be worth 3.333g each.
This means I can buy the top tier of herbs (Whiptail and Twilight Jasmine) for the going price of one Inferno Ink + 20g. If Inferno is going for 40g I can spend up to 60g per stack.
For the lower tier of herbs, I can spend half the price of one Inferno Ink + 20g. The Blackfallows are actually costing 4g each since I only get 5 of them per stack but using 20g here too keeps things simpler. Assuming the same 40g price for Inferno I can spend up to 40g on tier one herbs. A 20g difference between herb tiers.
Inferno Ink 30g then 30 + 20 = 50g and 15 + 20 = 35g for a difference of 15g
Inferno Ink 50g each then 50 + 20 = 70g and 25 + 20 = 45 g for a difference of 25g
Obviously the difference is half the price of one Inferno Ink. This leads to a simple rule of thumb I follow when buying herbs to mill. The difference in the value of the herb tiers is equal to 1/2 the value of an Inferno Ink. If I'm selling Inferno Ink in bulk for 40g then I'll buy top tier herbs for up to 60g a stack and lower tier for up to 40g a stack. If I'm only getting 30g for bulk Inferno then I'll pay up to 50g for the better herbs and 35g for the lesser herbs.
Of course these are my maximum prices, I try to get the herbs at least a little cheaper and only go all the way to the maximum if I'm short on materials and really need to make more glyphs right away.