This is based off of your charisma...the higher charisma you have the better deal you will get. However friends, charm person, nor the skill check bonus spell seem to have any effect, which I hope is a bug.
So have your highest charisma character initiate dialogue with the vendor for the best deal. 16 charisma is 45% listed price which is way off, but assuming there’s other modifiers you can get/accomplish to increase it, it maybe correct.
