Just for humor sake I checked Best Buy's trade in calculator for the A&TT S4 Black in "good" condition and fully reset, they offered $75. For $75 I would keep it as a spare phone or gaming or media player for kids in the car.
Yeah if you go the Best Buy route...obviously the phone is worth much more on sites such as Swappa. I am living proof of that. If my phone had only been worth $75 I would have kept it too.