My first tip on this site. Dont think I've seen it anywhere, maybe because it is so simple? Any how it has netted me large amounts of gold in the last two expansions.
In wrath it was saronite and khorium and in cata it has been pyrite and elementium (hardened mostly, HEB) that has given me most income.
I dont know how the prices can differ so much but often one can do atleast 200-300% on HEBs on my server. I never do it unless it is more than 100% profit.
Just get a feeling for what prices are good and not and you are set. Buy all ore when it feels right, sit on the bars if you think the price is to low or just buy them all out as well.
So all you miners out there, dont forget this its to good to skip.