it not only varies from server to server, but it also varies from day to day within that server. So if I have a stack of xxxx ore, I will check the AH on how many stacks of ore there is and what they are selling for and compare it to how many stacks of the bar there is and what is it selling for.
One day, I had 9 silver ore. Looked at AH and saw a ton of silver bars and zero silver ores. I put up that 9 silver ore for 20G and it sold in less than ten minutes. About a week later, I had 6 silver ore, took a look at the AH and saw only a couple silver bars selling well versus a bunch of silver ore selling for a little less. So I turned it into bars and sold for a few gold more than the ore.
What I am trying to say is, if you want to make money, take your time and look around a little bit in the AH. If you blow in and blow out expect to make a little less.