If you are adding a new line, it's paying the addition $9.99/line plus the $30/month data plan. If you replacing an existing phone and they allow you to replace it and renew your contract for another two years, it's the same as with any other phone other than adding the data plan.
So yeah, I just got my plan into a new family plan. Extra 40 dollars a month as fitz has said. Apparently you don't have to be family to be in the family plan, so yeah the AT&T had a chuckle when I didn't know that.