Do yourself a favor and go check the prices on both darkmoon decks and the cards that comprise them. Cards are going for 1k to 1500g on my server and the decks are over 15k. I buy any card i see under 1k and any card i need for up to 2k, and I'm still making money just spending 5-10 minutes a day browsing the AH for cards to make decks with. Also if there's a cheap card that you don't need but someone else does, buy it and trade it to them for a card that you do need.
The fair ends this weekend though, so get cracking.