I have been playing this game from its inception and nothing throws me more of a curve ball than discovering some BASIC feature that I really should have been aware of.
It is not too different to the Blacksmith hover wake-up call I had where you can see what their imbue skills are by hovering over the bottom left icon on their popups.
That actually took me a long time to suss out as well 
This feature is with the Vendors. On starting new characters I noticed that I was paying 150 for Tonic of Mending, yet some of my other characters were paying various amounts all the way down to 120.
It boils down to your reputation with that vendor’s faction, and this dictates what prices you pay, and it is not just potions, but EVERYTHING they sell.
There is a DISCOUNT percent they offer that can be seen by hovering over the top left icon of their popups.
Some examples from game …
Small issue, but annoyed I had not discovered this earlier.
A first time cash strapped player starting with a single character and having issues with purchasing potions for iron would benefit by knowing that the potions and items are CHEAPER at the vendors where your reputation is better.
Live and Learn - never ceases to amaze me.