If you are with ATT, under their Nextplan, you will probably have to pay for the additional line, plus the cost of the phone.
As it stands now, you can either pay for the phone upfront, which can be very pricey, or you can make monthly payments on the phone until it is paid off. Depending on the iPhone you choose, it will add another $20-25, more or less, to your monthly bill above the cost of the additional line. If you do not want the cost of the iPhone on your bill every month, you also have the option of buying a good used unlocked iPhone that you could bring to AT&T or any carrier. If you buy an iPhone directly from Apple, you will still need a provider, such as AT&T.
You used to be able to go to
Walmart and pay $100 for an iPhone, but those days are over. Most cell phone providers no longer subsidize the cost of phones.