Unfortunately, to get the most value for your system with a trade-in you usually have to do it during a promotion, and usually it's for credit towards an Xbox One (which you already have) or a PS4. Without promotions the systems don't seem to be that valuable for trading. Worse yet, most of the promotions require it to be a 250 GB hard drive, which yours isn't, so those won't help.
Are you selling any games with it? The best option you may have is to try and sell it on something like Craigslist. With the school year ending about now high school and college kids may be looking for a cheap time waster to buy. Look up the value of the games, then try and come up with a reasonable price, while factoring in the system as a bundle for a lot. If your system is in great condition you could probably ask for $60-80, along with adding more to the price for the games you may include with it.
Gamestop, Amazon and Best Buy do offer trade-ins, and you can go to those respective sites to look up values offered also.