How much does a Contract from ATNT cost monthly to pay with an IPhone for one person?

How much does a Contract from ATNT cost monthly to pay with an IPhone for one person?

One phone:
With ATT the cheapest smartphone contract plan is $50 for unlimited talk & text, plus $20 for 300MB data - so $70 minimum per month.
Having 1GB of data costs $85 per month.
Having 2GB of data costs $95 per month.

Kate's answer is right on. If you are not bound by a contract I suggest you go with a prepaid plan.
$40: 500 minutes talk, unlimited text.
$50: unlimited talk and text.
$60: unlimited talk, text and 2GB of data.

Save yourself some money and go with a prepaid plan. Taxes and fees are included, so these are flat rates. If you want to save even more check out Redpocket and Metro PCS. You will have to have your iPhone unlocked first.