You are renting your cards for WAY too much money. I've already seen this twice. 5 or 8 days of renting you spend the same amount of money that would buy you the cards!
You can rent most of them for 0.1 DEC per day, yet you pay 97 DEC per Day.
Please check out peakmonsters.com - it's a 3rd party website for card management, rentals and a lot more. It will help you get best prices and not be screwed by someone.
Edit: I just learned that peakmonsters displays set rentals in a wrong way. You pay 97 DEC/day for the entire set?
So I got 3.5k coins after I purchased the spell book. I used 1k of the coins to rent the cards for the rest of the season.