Data is charged per phone. So if you were to join their family plan with an iPhone it would be $9.99 for the extra line, then $15-30 for data depending on what data plan you get for your phone.
I'm sorry, one more question- so just to confirm, you do NOT have to pay individual text plans with the $30 Family Text plan on the bill? Just for the extra line, data plan and all other services are included in the regular monthly total?