I've made so much gold by making cheap Darkmoon Faire decks. How you ask? It's actually really simple, simpler than you think.
I've made a total of 4 decks, three of them being Volcano Decks, and one Hurricane Deck. They've all been made for under 10,000g, with the lowest being 8300g. I make them for cheap and sell them for high. Out of the four decks that I've made, I've sold the three of them for a combined total of 69,000g, do keep in mind that I did keep/use one of the Volcano Deck on my mage, thus cutting down on profit tremendously. By selling the three left over decks, I gained a total profit of 42,500g.
How does it work?
I told you it was simple didn't I? Well, It is. I buy the cards off the Auction House. I check the AH twice everyday for cheap cards and I check both factions. I've managed to get cards as cheap as 700g by doing so. Once I have a full deck completed, I make it and sell it in trade chat.
I've built decks for as cheap as 8300g ( 1037g per card ) and I've sold them for 22, 23.5, and 24k. It's so easy, it just requires about 10,000g and some patience. Patience being the keyword, The few decks that I've built have taken me about a week to get. Why? Because I'm waiting for the right price to buy the cards that I need. Some cards might take longer to obtain than others, but you simply need to have patience.
Try it out guys, slowly build a deck of your own ( DON'T Pick the earthquake deck - It sells horrible ) for cheap, once you've got the deck complete, sell it for a profit. It's easy, simply & a very effective method of making gold.