How Much Would The New Iphone Cost Monthly If I Added It To My Current Family Plan

I really want to get the new iPhone but I dont really know how much the extra data plan costs each month or how much it costs to add the new iPhone to my already existing plan. Can someone tell me the cheapest data plan I can get for the new iPhone and how much it will cost a month, along with home much it will cost to add an iPhone to my existing plan?