I like Durotar for copper. It's not a huge zone and once you get to know the spawn points, it's fairly easy to loop around the zone. There are also a couple of caves with copper in them as well.
Mulgore isn't bad and since almost all of the rocky areas are around the edges, all you have to do is run laps around the zone. There is a lot of copper in that Venture Co mine and that mine is huge inside if you are high enough level to go through the whole place. There is even another mine exit to the bluffs above where a few more copper nodes spawn.
Copper on the Horde AH on Cenarius (an old high pop server) goes around 2 gold a stack right now. Copper on the Alliance AH on Cenarius goes for 50s to 1g50s a stack. I have always found it a lot easier to make gold on the Horde AH than on the Alliance AH.
Prices on the Horde AH on Tanaris (a newer med pop server) are much lower at around 1 gold per stack last time I checked. (I don't play those alts as much.)