Someone needs to lock this thread down, since it's been asked before, by OPs who do not fully understand the capabilities of the Nexus 7.
1) The Nexus 7 is a wifi-only device. This means it's not capable of retrieving data from the internet on its own. Wifi-only devices are reliant on connecting to an exsisting wifi connection via wifi hotspots to retrieve data from the internet. This includes mobile hotspots (aka wireless tethering)
2)If you are intrested in wireless tethering, whether you pay for the service (eg Verizon mobile hotspot) or not (eg Foxfi app) it's important that you keep a record of how much data you are consuming from your hosting device, which is most likely your cell phone, and install a data management app that tells you how much data you've consumed from the start of your billing period.
3)If your phone has ICS already installed on your host device, you don't have to install an addition management app. Just go to Settings>Data Usage> and adjust the setting to your prefrences. You can even set an alert and limit on your data usage. Feel free to check on your usage as many times as you like.
I hope I was able to clear up any confusion, and did my best not to sound condecending. If you have any questions about this matter, please PM me.