It's definitely not cheap if you want to own your cards, that's why renting your card works if you manage them properly. The DEC return from battles is currently broken, so hopefully that gets addressed once CL is done. That being said, the new reward cards are very very affordable, so owning those and combining with some rentals would work. This game is not for the impatient and new players need to understand that. Its not Axie whereby you are almost able to calculate your ROI at the start.