Ever look at the information on Peakmonsters about card rentals. You can find some interesting tidbits.
My personal favorite is this one :
Gold Cave Slug lvl 7: Monthly ROA is 116% which means more than double your money after only 30 days!
Now its nice to be able to use Peakmonsters to see what cards are available and in what demand they are. However, don't be fooled by the numbers, always dig a little deeper.
A little closer look reveals this :
There is only one lvl 7 card for rent at 1,200 DEC but with a lvl 8 available for 108 DEC only a fool would rent the more expensive one. It's true that at 1200dec/day you'd pay off the card in a month but don't let a quick look at the numbers fool you. Do the math to see if the deal is good or not :)
Happy Hunting!