Hi - 300/300 herb/alch. A lot has already been covered here, but I'll add my 2c with as little repetition as I can.
As an alchemist, your bread and butter will be arcanite transmutes. Look at the AH for the prices of arcane crystals and arcanite. On my server I can buy an arcane crystal for 21G and sell an arcanite bar for 30-31. Thats 9-10G profit every 2 days. I tend to make more money this way than other people I know that do the transmutes directly. I don't mess around with the other transmutes (metals) unless you got the life/undeath/air/etc transmutes, which can also bring in cash (but the recipes are expensive/rare)
Secondly, I find that herbs generally turn a better profit than potions. Not sure why this is, but it's what I've seen. The one exception may be major healing potions, which sell for about 10G a stack on my server. For best profit, I tend to split my herbs into stacks of 5. I find it easier, for example, to sell 4 stacks of 5 at 3G each than to sell 1 stack of 20 for 10G. (the one exception, for some reason, being dreamfoil, which tends to sell well in stack of 20, probably because the major mana potions each require 3 of them).
In general, I don't go out of my way to go herb picking, its not really an efficient way of making money. But always pick up what you see while you're grinding. For example, at 300, sungrass might not be of much use to you, but that level 40 alchemist who can't get to the hinterlands without being split in half will pay good money for it.
Pretty much any high level herb will sell well, the best I've found are Dreamfoil (major mana pots), plaguebloom (both very plentiful in plaguelands and felwood), golden samsan, mountain silversage (a little harder to get). Each of these go for upwards of 10-12G a stack.
Also, gromsblood and fadeleaf both sell very well, stack of 5 for about 3-3.5G. Ghost mushroom sells for about 1G a piece, but you have to go out of your way usually to get it.
Paladins require arthas tears for their epic mount, so this also sells well. I find felwood to be better than plaguelands for this herb.