One source for learning more about market prices is ebay and amazon. You can search the herbs you are planning to
sell (though this is more for dry volume, it can give you an idea.)
The main thing I wanted to post though is that Comfrey is sometimes illegal to sell (or even advise to use) in some jurisdictions. I've talked to herbalists in Ontario who are restricted from even saying on the record that it can be taken internally. They themselves consumed it casually as a minor part of diet (slip a leaf or two in a smoothie, for instance).
The reason for this seems to be similar to the reasons sassafras is a bit controversial. Research will be done, and rats will die when fed enormous amounts (that would be equivalent to us eating a bushel at a sitting). Alternately, some studies will isolate a chemical component from the herb, and
feed that exclusively to rats.
Not to fully discount the science, sometimes studies will also look at populations who consume a lot of the herb to see if there is anything that makes them stand out (eg. higher incidence of cancer).
Personally, I would tend to trust traditional usage by peoples who have consumed these things over many generations, as opposed to studies that seem ill-designed (or designed for another purpose - such as isolating a chemical for future synthesis in a pharmaceutical.)