Just another quick check I did, that same armour thats 1500 gold at 15 persuasion is 3100 gold with -1 persuasion.
You can only get 15 persuasion in Act 3, and the illithid expertise as well needs the astral tadpole at the end of act 2, but even before then I had around -44% or so discount.
I didn't realize until this thread why it shot up to -76% in act 3, which was due to just clicking on every tadpole power on my MC.
