Now the dust has settled there are quite a few cards up for rent for good prices. What is a good price is of course debatable. I would say below 5%. Summoners being the main staple of any team means they are the most important if you want to rent a set. A max Malric Inferno will cost you $0.06 per day (Alpha and beta). It is worth noting sometimes alpha cards maybe cheaper to rent, so have a check.
Looking at the other cards in the fire set is cost just $0.18 per day for a max basic beta set (no legendary) or and extra $0.077 with legendary. For $0.021 you can rent the two rare fires promos max for $0.021 per day.
Problems at present:
-Many cards are on cooldown, so you have to pay for them for a few days and you can't play them.
-Batch lending options should be available, as its arkward to rent a full set atm.
-Some cards aren't even available to rent e.g. exploding dwarf max level.
-You lose money as you don't own the cards, so after the period you still don't own them. While you can earn some back by daily quest odds are you will pay more than the extra few cards you have.
-You need to rent a couple high level sets and the expensive legionaries to have a chance at the higher leagues.