I usually just grab whatever the highest CP/DEC cards are that are available on the market, which you can view on PeakMonsters. I've been able to consistently rent cards for around 300-400 CP/DEC and every once in a while get lucky with a better deal. But even at 300 CP/DEC, you're only looking at 50 DEC/day to rent the 15k power needed for silver 3.