As part of a trade-in promotion that kicks in Friday when the new iPhones (5c and 5s) become available and runs through Sunday, Best Buy wants to give you the 16GB 5c model for free, in exchange for your current iPhone 4 or iPhone 4S. Of course, “free” doesn’t last long in the wireless business. Would-be customers must be eligible for an upgrade, and commit to the customary two-year contract with Verizon Wireless, AT&T or Sprint. You’re also on the hook for taxes and activation fees. Best Buy says that if your current iPhone 4 or iPhone 4S is worth more than the $99.99 cost of the iPhone 5c, the retailer will issue a gift card to make up the difference.