Nice. As you can see that's the model I have. The problem is my upgrade time comes about when the rumors have an iPhone 5S coming out, which probably means the price will drop.
HOWEVER, I finally convinced my mom to upgrade her iPhone 3GS. Well, when I say convinced, I basically said "Gimme your phone. I'll bring you back an iPhone 5." I did this once I found out about the trade-in program and valued her iPhone 3GS 32GB. $175. I could've gotten an extra $5 for the cable, but those are worth more than $5 when you, your dad, half your aunts and uncles, and your sister and her husband all have iPhones and iPads that use the old cable.
The iPhone 5 16GB was on sale for $150. Bought an extra Lightning cable and then a car charger for Lighting -- which she has since lost, argh -- for $40 total. So she basically upgraded and got two cables for $25, give or take a few in taxes.
As some people say, you can probably do better on eBay. But I'd rather take the easier road for an assured $175 gift card that I can turn around on a phone. Had I done this in the correct order, as the OP said I would've gotten another $50. But I didn't. I used it on a vacuum cleaner the next week.
Very good program that not near enough people have heard of.