The Best Buy free upgrade is not as free as one would think initially. There are plenty of accessories to purchase in order to move from an iPhone 4 to an iPhone 5.
The base price of your iPhone will be based on your carriers eligibility for an upgrade. For me that was $150. Once Best Buy gives you $150 in a trade allowance the net cost is zero, however, AT&T charges $36 for the upgrade processing fee and you probably need an extra power cable or two and if you need a new case and even a screen protector it adds up very quickly.
Free is not so free. But it is somewhat cheaper. Especially if you are in the market.