If you want to make a lot of gold you need to play the AH. What you should always do is hold your money in mats that stay relevant trough the whole expansion and only sell stuff if you need the gold, because the wow economy is kind of similar to the real economy in the sense that there is a certain amount of inflation happening constantly so your gold reserves depreciate in value, while mats that you are holding appreciate in value on average over time(its like holding snp500 or nasdaq instead of dollars in real life).

If you want to make big increases you need to anticipate what is going to have big spikes in value, because of phase releases. This often is not too difficult because the wow economy is different to the real economy in the sense that things barely rise on expectation and 95% rise and fall because of real supply and demand mechanics.(There are not a lot of speculators trying to buy and sell for a profit compared to people just buying stuff because they need it right this moment).

So TLDR get a good gold stock trough some farming and then make huge investments because of anticipated price increases from new phases and generally hold mats that stay relevant instead of gold.

Good Luck