You are totally right, rental income for CL is low. But, I was playing Silver I league and earning nearly 100-200 Dec daily so I decided to rent out all my cards. Today ( 2. day of the season ) I earn 350-400 Dec/Day with my 1.700 USD valued cards. At the end of the season, this figure rises up to 2000 DEC or more.

it's not much, but the daily rental income seems more profitable than playing games, as there are not many good cards coming out of the daily prize chests.

I buy cards with daily rental income and prepare for the next game modes.