Here’s a nice comparison chart from Cult of Mac showing that you can save about $700 over the 2 year cost of an iPhone 4S by choosing prepaid. The cost savings would rise to $1,000 if you go with Virgin’s lowest cost $30/mo for 300 minute plan.
One caveat – prepaid phones typically don’t roam. We often take cell service for granted and due to roaming agreements aren’t even aware of some occasions when our service is being provided by a roaming partner.
Another option not listed is Straight Talk which provides for $45/mo unlimited voice, text, data although you’d have to bring your own unlocked iPhone and the data is strictly capped at about 2 GB per month or 100 mb per day.