Herbs are priced per pound, but aside from Belladonna they don’t mention how much a single treatment consumes.
I’m /guessing/ the intention is that 1lb is one dose – even if it seems a bit much… 1 pound of dried leaves is a lot of leaves. 
1 dose is 1 pound.
Having never seen any clinical research on the use of belladonna to cure lycanthropy, I cannot speak to whether I have over-estimated the dosage. For game purposes, 1 dose = 1lb is clean and workable.
I imagine 1 lb. makes sense once you factor in mashing it up in a mortar and pestle.