AT&T Family plan?

My mom has an AT&T phone and I found one I liked. We thought about doing the family plan but we don't know how much it'll cost. All I'd use the phone was for seldom calls, seldom texting but using Windows Mobile Live most of the time. She seldom calls and never texts and she doesn't have Windows Mobile Live. The phone I want is $99.99 finally a phone I like and isn't a stupid pay as you go. So how much would we be paying if we did the family plan? If you need to know more email me.