Hey Nancy, this is a great question. I found a few online sources that offer some yield information.
1. New Crops Organics [LINK] offers dried yield information for skullcap, nettle, echinacea, etc.
2. Book: Farming a Few Acres of Herbs [LINK] offers information about yields and pricing.
I know this isn’t exactly what you were looking for but hopefully, it’s a start to finding the information you’re seeking. If you find another resource, please pass it along.