YouBuyRite via eBay offers the refurbished Unlocked Apple iPhone 4 16GB GSM Smartphone in Black, model no. MD234LL/A, for $124.95 with free shipping. That's the best price we've seen for an unlocked 16GB iPhone 4. (It's a current low by $75.) It's compatible with GSM networks such as AT&T, T-Mobile, and Vodafone; it can't be used with CDMA networks such as Verizon or Sprint.
Note: No warranty information is provided and it cannot support iOS 8.