Right now with AT&T our bill is $140 and the wife and I have unlimited data, my daughter only has 200MB but that's OK, she is only twelve and even under our shared minutes of 700, we have plenty to go around with free weekends and mobile to mobile.
A 12 year old with a smart phone? I must be really out of touch with kids, because I'm 28 and don't even have a smartphone with a data plan because it's just too expensive. But that's just me, to each their own.
I don't really see who this shared data plan is designed for. A family of 10 that all have smart phones, but never actually use the data so they can all share a gigabyte?