yes you can do that. It costs $30 for internet which is required by at&t for the iPhone. Plus it costs an extra 10 a month for your own phone number but the price of the plan does not increase.
Right now the cheapest iPhone is the iPhone 3G for $99 with a 2 year contract agreement
http://www.att.com/wireless/iphone/
Right now the cheapest iPhone is the iPhone 3G for $99 with a 2 year contract agreement
http://www.att.com/wireless/iphone/