You don't need to buy good cards to get started. In fact, you can rent cards and earn the DEC needed to start buying them. Check out @mozzie5 and his current challenge here. It will give you an idea of what you can accomplish without buying anything.

You earn DEC and get free cards in the quest rewards as well as the season rewards at the end. It will take awhile but you can play, earn money, and build a deck without spending anything but time. Good luck!