At Sprint, trade in any Apple iPhone and get $100 off an Apple iPhone 4S when you signup for new service or add a line of service. That drops the price of the least expensive iPhone 4S (16GB model) to $99.99 with free shipping with new service. That's among the best trade-in offers we've seen for an iPhone. (We saw up to $263 for trade-in of an iPhone 4 16GB for AT&T last fall.) Reserve your new iPhone 4S by July 1 and activate it by July 3. Trade in your old phone by August 14.