It's hard to balance the drug prices. When you lower / make one higher, you have to lower / higher a different one, ect. It will never be fully balanced, but what you have to do, is simply calculate which would be easiest to grow and sells the best compared to how you get the drugs yourself. Compare price to the difficulty of growing them. Find the best option.
For me, even though I have found it takes long to plant, nether wart is a great one because its got a good price considering you do not have to cook them, and you can get bigger quantities of them. Personally, the only problem is the time it takes to replant. However, this is the same for wheat potatos and other soil drugs.
The real downside, is that you walk slow on soul sand, meaning it does take longer to plant. To combat this, I suggest you get your herbalism up in MCMMO, as it will plant some of the crops automatically without you needing to. In theory, that would mean if you have a high enough level, you would only need to harvest and not plant them, as, with a high enough level, it would plant it all for you. However, you would have to be a VERY high level.