If you frequent any of the Warcraft forums (official or fansite), you will constantly see threads pop up asking for gold making advice. The most common answer will be to get a gathering profession and preferably Mining. Whilst this is good for a new player or someone who is extremely bad at managing their WoW finances, it's not really good advice. Mining is a solid profession, with a guaranteed income but it will not lead to your riches.
The economy in WoW is build on inflation/deflation (increase/decrease in prices), which is very cyclical i.e. the economy rises and falls depending on the timeline of the game; how close it is from a patch/expansion. There are other factors such as realm population & supply/demand issues, but they're less relevant in this case. If you simply judge Mining on inflation, it's not a great profession. Let's just look at some basic stats from within the game to explain my point:
Stack of Fel Iron & Adamantite ore during TBC; 20-30g
Stack of Cobalt & Saronite ore during Wrath; 20-30g
20-30g might sound like a good price, but considering the value of gold today compared to 2 years ago, it's not so valuable. The biggest gold sink in Burning Crusade was the 5,000g for epic flying, today it's 16,000g (after rep) for a Tundra Mount. In simpler terms, to pay for my flying I would have to sell 200 stacks of Ore at 25g a stack, but to buy the Tundra mount I would have to sell 640 stacks. I realise this mount is less vital than epic flying, but this is just a simple example to illustrate the deteriorating value of your mining. Even basic daily costs like repairs or flight paths will need you to sell more ore to cover.
This diminishing return seems even worse when you consider the alternatives. Many other professions have seen a rise in auction house prices (herbalism and cooking for example). Even daily quests offer more gold than they did in the previous expansion. So in relative terms mining is no longer as valuable, and this is a trend that will likely continue.
I'm not suggesting mining is a bad profession because I have it on my main, but you'll have to think outside of the box if you want to increase the cash flow.