There is nothing about determining costs because the idea was for players to looking and gathering them as they travel, not purchasing them.
And because it completely slipped my mind...
Here are some sample costs to get you going. Remember, trying to sell an herb will likely result in only 50% of its sale value to a shopkeeper (and local guilds won't like freelancers trying to move in and take their business)
For Enchanted Herbs, use the normal costs.
For other Class A Herbs - 10+ gp per dose
For Class B Herbs - 2-20 sp per dose
For Class C Herbs - 1-10 sp per dose
For Class D Herbs - 2-20 cp per dose
For Class E Herbs - 1-5 cp per treatment (i.e. enough doses for a a full course of treatment)