If you are looking for new cards and you'd rather not spend real money investing in card packs, take a look at the market. There, you can buy and sell cards at closer to their actual value. Be patient. Look around and buy a card that is going for a bit less than it probably should. Buy it and then list it, but at a price more in line with the stats (while still keeping it low enough to appeal to players). Keep repeating the process, making a bit of profit each time, and you can start to build up some reserves that allow you to invest in the more expensive cards that will build your team.