Sounds like you are doing pretty good.
I pay $10/month for each of my parents cell phones through Ringplus.
I've got my wife on a $30 unlimited everything plan on Replublic Wireless.
I know this is more than your current expense of $0, but $10 a month for a phone with data can pay for itself in ways such as finding cheap fuel with the gasbuddy app, navigation, and consolidating trips by asking if anyone needs something while you are already at the store.