Herb Yields

We are looking for "anticipated yields" for the following herbs: St Johnswort, comfrey, calendula, chamomile, Echinacea angustifolia and Echinacea purpurea, mugwort, red clover, lavender, garlic and Astragalus.
We are well aware of the myriad of variables affecting yield. So our premise would be that we are spacing the herbs for commercial harvest, we have the proper soil, sun, water and temperature conditions for the plants to thrive and we will have minimal pest damage. (Wishful thinking...)
So, as a starting point, is there a "rule of thumb" for yields? Is there a reference book you could send us to look at?

There is no rule of thumb that I know of for estimating yields for herbs. Also the information is scattered over many publications and I cannot recommend a definitive tome on the subject!
I was unable to find any data for Echinacea angustifolia, mugwort and Astragalus.
St Johnwort: 2500 kg(kilogram)/ha(hectare) of dried herb per year.
Comfrey: 11400kg/ha of dried leaf, 1800kg/ha of dried root
Calendula: 6000-9000kg/ha(fresh flowers), 1200-2250kg/ha(dried flowers)
Chamomile, German: 3500-8000kg/ha(fresh flowers), 350-800kg/ha(dried flowers)
Echinacea purpurea: 1000-1360kg/ha(yr 4 dried roots), 6000-10000kg/ha(yr2-4 dried herb)
Red clover: 1800kg/ha(dried? flowers)
Lavender: 3000-15000kg/ha(fresh flowering tops), 300-1100kg/ha(dried flowering tops)
Garlic: 1300-29000kg/ha(fresh)
N.B. 1 kilogram (kg) equals 2.2 pounds; 1 hectare (ha) equals 2.5 acres.