Peakmonsters is a fantastic tool to see if you are getting a fair price when you are renting cards from other players. If you go to My Cards and compare your rentals you can see at a glance.
Now you can quickly look for red arrows. If we look at the goblin sorcerer below we can see that the cheapest level 2 on the market is 0.194 which is 1.11 times cheaper. For a cheap card this doesn't really matter as we are only talking about a tiny amount of dec so you could keep renting this one. If we look at Cerberus below you'll notice a green arrow indicating we are getting a fantastic bargain as the cheapest one on the market is 3.87 times more expensive. Hooray ! Keep renting him for sure.
Here is another example, I am renting a Venari Heatsmith that is now 5.1 times cheaper, I wont be renewing him in 7 hours time.
Note in the example above I've demonstrated checking cards you have rented from another player. It can also be used just as effectively to keep tabs on cards YOU are renting to OTHER players.
I wish you Happy & cost effective renting :-)
