Ore bought from faction vendors costs 8gp/purchase, however each purchase has a random chance of being any one of the different types of ore. I tested 40 purchases it ended up costing me approx 9.41gp/ingot.
take the following things for what they are. just wanted to record these here as well. also arrows are currently 8 gp they sell for 7 gp bolts are 8 gp they sell for 7 gp boards are 6 gp they sell for 3 gp bottles are 5 gp they cost 4gp at alchemist shop (mage guild discount)