Guarantee Cellular via eBay offers the refurbished Unlocked Apple iPhone 4S 16GB GSM Smartphone in Black or White for $59.99 with free shipping. That's $10 under our refurb mention from last month and the lowest total price we've seen for an iPhone 4S 16GB Phone in any condition. (It's a current best by $12.) It's compatible with GSM networks such as AT&T and T-Mobile.
Note: No warranty information is provided, although the seller does offer a 30-day money-back guarantee.