Would It Cost More To put an extra line on an AT&T unlimited family plan?

Well we have been with AT&T for a while, and we still have the unlimited data, unlimited text and unlimited call. So I was wondering if it would cost a lot more to add one an extra line. The plan only has 2 GoPhones and 2 SmartPhones on it. So could one of the GoPhones be replaces my an iPhone? ( GoPhones are $10 a month )

I don't think it will and if it does I don't think it will cost much