screenshot by Lance Whitney/CNET

Those of you who want to shed your old iPhone in exchange for a new model can score a trade-in deal at Best Buy this weekend.

From now through Sunday, consumers who trade in an iPhone 5, 4S, or 4 and upgrade to a 5S or 5C will get up to $200 and more in credit toward the purchase of the new phone. Based on Best Buy's iPhone prices, that means some customers will be able to snag a new iPhone for free.

First off, here are some of Best Buy's current iPhone prices:

The 16GB iPhone 5s is on sale for $100 (half off the usual cost), but only for Sprint and Verizon subscribers. The phone still sells for $200 for AT&T customers. The 32GB and 64GB iPhone 5s handsets are currently $100 off, but again only for Sprint and Verizon customers.

The 16GB iPhone 5c is selling for $50 (a $50 savings) on Sprint and Verizon. The 16GB iPhone 5c is $100 on AT&T, though the white model can be had for just a buck.

Okay, and here's how the trade-in deals play out, according to Best Buy:

AT&T, Sprint, and Verizon customers who trade in an iPhone 5 can get the 16GB 5S or any 5C for free courtesy of the guaranteed minimum $200 gift card.

Sprint and Verizon customers who trade in an iPhone 4S can also get a free 16GB 5S through a guaranteed minimum $150 gift card. AT&T subscribers would have to kick in an extra $50 for the phone.

Sprint and Verizon customers who trade in an iPhone 4 can also snare a free 16GB 5S via the guaranteed minimum $100 gift card. Those on AT&T would have to add $100 to get the 16GB version.

Got all that? Wait, there's more.

The prices listed above naturally require the usual two-year agreement with the carrier of your choice, so you must either be a new customer or eligible for an upgrade. Verizon Edge and Sprint Easy Pay subscribers can also get a trade-in credit when they swap their older iPhone for a 5s or 5c, though the new phone may require a down payment, according to Best Buy.

The phone you trade in must be in good working condition, meaning it has to power on, have no water damage, and sport no cracks in the screen. But assuming the phone meets those conditions, Best Buy promises a guaranteed minimum trade-in value of $200 for the 16GB iPhone 5s, $150 for the 16GB 4S, and $100 for the 16GB iPhone 4.

And what if your phone is worth more than the cost of a new 5S or 5C? Best Buy will make up the difference in the form of a regular gift card. The retailer's trade-in pricing page can help you determine the value of your phone.

The deal is good only at Best Buy and Best Buy Mobile retail stores in the US that accept trade-ins.