i have never seen such figures in rpg supplements and related books.
but medical substances are usually rated in milligrams with super-heavy compounds ranging into the 2-5g (2000-5000mg).
given that the compound to dead-mass ratio in plants can range from 1/10 to 1/1000.
given all that uncertainty ... you can readiely assume that an unrefined herbal dose of medicine has a weight around 3-20 grams (1/10 to 1 oz).
if you would ask me as a GM, i would rule around 1-2 oz. for simplicity and imperialism.