Looking to buy a brand-spanking-new iPhone 5S on the cheap? Your best bet might be to go somewhere you havent been in a long, long time: a RadioShack. The struggling electronics chain has a new promotion in which you can trade an eligible iPhone 4S for the fancy 5S. Crazy, right? Its not exactly straightforward, but heres how it works. Normally, an iPhone 5S with a new or renewed contract costs $199. Starting Friday, RadioShack is knocking $100 off the price of a 5S with contract. Then, if you own an old 4S in working condition, you can get a