EBay, swapa, Craigslist, these are areas you could check and see the approximate price for your device... it does vary and is dependant upon different things... condition, age, accessories, and if the successor to that model is due to be released... hope this helps.
I typically check completed listings on eBay before going on craigslist, but looks like movaluate already does that for you. priceonomics isnt too bad but there prices are more ballpark where as movaluate's seem pretty precise. swappa's nice too especially if you are trying to sell or buy all in one place.
I register the forum because of this thread. I always curious about my phone's worth when my contract ends.
Movaluate guides its price which is come from ebay complete sold price.
If you think ebay is the most fair market, Movaluate.com provides the most fair price.