If you have purchased any items(not DLCs) BEFORE Trading Cards addition, even just $1 one and already sold it - you would get 3 card drops out of 5. To get them you have to spend approximately 5 minutes per card in the game.
For new players.
By Steam rules everyone is eligible for 1 card drop per $ 9 spent on items from "Support Bloody Walls" section. Unfortunately, Steam does not include payed DLCs in that sum as far as i know, but it's possible to craft Trading Cards booster pack from gems if you wish.
For more details about Steam rules see here, part about "free to play games": http://steamcommunity.com/tradingcards/faq