Hey, if you play enough you can offset the DEC costs of renting pretty easily.
There's various approaches on how to rent, but I tend try to get cost effective cards that I actually use, especially gold cards since they give a great amount of power but they also increase your DEC earnings when you use them in battles.
Your should start aiming for 15k power to stay in Silver III since you you also get 2 chests per daily quest in that tier.
Good luck!