Looking at the astronomical prices of the Yodins and the llamas can be a dizzying experience especially for newbies. But, you can build a decent card collection from renting/buying some low cost high payoff cards. The following is not financial advice.
Buy - Sandworm - Although a slow card, if positioned correctly, it can clear the enemy backlines effectively.
Rent - Lord Arianthus - At an average of 24 dec, its a bargain card that is the ultimate tank.
Buy - Kelp Initiate - If you play the water deck, this is perfect for low mana matches.
Rent - Robo- Dragon Knight - At an average of 5-6 dec per day, this tank will be the star of any lineup.
Rent - War Chang - A high damage neutral card with retaliate at just an average of 2-3 dec a day, huge payoff.