This Thursday, Radioshack, Best Buy and Walmart will begin to sell the new iPhone 4. If you're planning on purchasing one and already have an iPhone 3G or iPhone 3GS, you can trade it in at these retailers which can cut the cost or even get it free.

"Under RadioShack's promotion, which runs through Saturday, folks can trade in their old iPhone 3G for $100 or an iPhone 3GS for $200 -- regardless of whether it's a 16- or 32-gigabyte model. That can go a long way toward slicing the cost of a new iPhone 4, which is set to retail for $199 for a 16-GB model and $299 for a 32-GB.

RadioShack has only three criteria for a trade in: The iPhone turns on, the screen isn't cracked and the phone has no water damage. The company's "Trade & Save" program allows customers to have their old iPhones and other electronic devices appraised, traded in and receive an instant store credit on a RadioShack gift card"

"Wal-Mart Stores appear to offer the next best deal, with its electronics trade-in program run in partnership with online consumer electronics recycling giant Gazelle. The trade-in program basically queries iPhone owners about whether their used device makes calls successfully, is free of water damage, is in good condition and has its AC adapter and cables. A 16-GB iPhone 3GS could potentially fetch a trade-in value of $158, while a 32-GB version could land $188."

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"Even though Best Buy takes trade-ins, its devices go through a more rigorous evaluation, and payouts can be lower than those that Radio Shack and Wal-Mart are offering. For example, an iPhone 3GS with 16 GB could fetch $119 and a 32-GB model potentially $159, according to Best Buy's "Trade-In Center" estimator".

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