Once you have maxed a champion the value of a cover effectively drops drastically as it is not giving you a level up + extras and is instead only giving 250 etc. iso.
Given that everyone is starved of iso, why not increase the sale value of additional covers for that character by 50 or 100%? This would be a welcome bonus and a suitable reward for a person having rostered, leveled and max championed a particular character.
Two star going to 500, three star to 1000 and four star to 2000 would clearly not be gamebreaking.