This is sort of a difficult skill to give training advice on. Basically with herblore you want to just check out whats selling good at the time and go from there. The prices of pots are always rising and falling so there is no definite "best way". Look for best xp first then check the cheapest of those. Most likely if you're training a skill and are dedicated youll pay that little extra cash to get "faster/better" exp.
I'm not a master herbalist, but that's my logical approach. I believe Shane has also recommended similar advice to listeners of the Update.
Hope this helps
