Graylo posted a comment looking for a mod to help decide the cheapest way to get the inks off the AH at the current market price. I take a different look at this. I feel that if you are buying herbs of the AH on the spot to make your glyphs you are over paying for them. It would be best to get a farmer to send them to you or just buy the herbs when they are low.
I am a firm believer in having a large stock of ink of the sea. Keep 7 days worth of ink in stock when you can. It is a buffer between you and high prices. It also gives you the supply you need when demand spikes.
I only buy the northrend herbs and use the ink trader for the other ink. Snowfall ink can be an issue for many scribes. You need to decide on the bulk value of it. Just because I see it on the AH for 18G does not mean that I can sell 6 stacks of it for 18G. The market value is around 15G so I supply it to my buyer for 13G each. I know at that price he will buy every stack I send him. Even when the ah dips down to 12G (from me driving it down), he will still pick it up. I send him 3 stacks a day so he does not even have to look at the AH.
Because I can sell my snowfall ink, I prefer the high yield herbs like lichbloom/icethorn/adder's tongue (but at the right price, I will buy the others too). When I had a farmer I made it clear to him that all I wanted was the adders at a set price. I would take the other two herbs only if it was for the same price. As long as I took his entire inventory, he was happy to give it all to me for that price. I do the same on the AH. Lichbloom will usually sell for 50-100% more then the other two. But when it matches the others, I will buy it just as fast.
Instead of deciding how much my ink will cost me on the fly at current AH prices, I set how cheap I will get it a long time ago. I look at my inventory to decide if I am paying too much or not enough for it. Sometimes I will buy out the entire auction house because of a undercutting war or farmer flooding it. It can be intimidating to buy that much at once, but after you do it several times you get use to it.
I can easily go a week without buying herbs off the AH. I only do this when prices are too high and in that case, I do let my stock run low. That is exactly why I have it. I don't build that stockpile of ink so that I don't have to buy it but once a week. I am constantly buying it and it gives me the ability to only buy it when it is cheap.
I do use 30 stacks of ink a day on average (100 stacks of herbs). I have to have a huge stockpile. Even if you are small time, you can use that 7 day metric as how much you should have in stock. You would be able to use a much lower threshold then I do (unless someone like me buys it as much as I do). I would like to say I pay less than market price, but I'm one of the major players that holds market price for herbs where it is.