I have to bump this because I found two very good trading card games. You can play competitively online and buy packs just like any other TCG. It's also manageable to play with little or zero budget. It's pretty easy to earn free points by winning.
Assassin's Creed: Recollections is the most intense TCG I've ever played. It's a realtime card game where you try to control two of three "regions" on a board. You do this by launching sites, agents, and actions. The UI design is the most beautiful goddam thing I've ever seen, and the gameplay is rock solid. The downside is that Ubisoft's servers are unreliable. They usually go down about once a day for an hour.
Shadow Era is your typical TCG akin to Magic and Yugioh. It's more simple to play and understand than Recollections, but it has a great formula. You choose one hero card and base your deck around their traits. If your hero is a Human Mage, you can only use Human ally/item cards or Mage ally/item cards. It makes the matchups pretty dynamic. The lore is a little over-nerdy like Magic, but it's still alright. It's too fun to be written off for that reason alone.
My username in all games is darkozl if you guys want to play. You should play.. please..