There is a quest you can do for Tashun in the Lost Mines(I think, or forgotten?). His reward is a 'Discount' on his items. What I am not sure, since I never really bought much from him to begin with, is that if this discount is a continuous thing. Is there some way to tell if this discount is continuous?
I think it is valid only for difficulty on which you have completed the quest,since you can do the quest again on other difficulties.
Note that I could be wrong since I didn't check prices before completing the quest second time.
Also if someone wanders the discount is 50%.