I did a lot of research on charging pads before I purchased an N7-2013. Price wasn't so much an issue (not that I have a lot of money), but I was more concerned with functionality and longevity. I ended up purchasing the CHOE UPGRADED Qi Wireless Charger Charging Pad. It retails at $70 but is currently on Amazon for a mere $22.00.
It charges my N7 through its slim case; and I don't have to be too careful about how I place the device on the charger, as long as it's fairly centered..
I also use the pad to charge my Moto G-2 phone and my N7-2012 (which sports a very thick leather case) with the use of the DigiYes Wireless Charging Receiver Modules, which plug into the devices' USB port.