My best guess is that the price seen while an item is still on the merchant's side of the screen is the base price. The modifiers for CHA, merchant attitude, and suchlike get factored in once an item is moved into the trade/barter section of the screen. I don't know that the 133% mentioned by Kingslayer is a constant or anything, but I definitely saw prices double, plus a bit, as soon as I pulled them out of the merchant's inventory.