Currently for sale as new models are NOT eligible for Trade-in. Trade-in Values offered will change from time-to-time, based on market conditions.
The answer to almost all questions like this is:
Use the Apple web site.
NB>> Set to the country where you want to trade it in. Each county may have different values.
-OR- use the list and links Limnos provided above
Go through the motions of specifying your device for trade-in... this is Apple, nothing becomes permanent until you give the Final OK, and you can back out easily at any point in the process.
In the US, you enter the serial number, and they use the serial number to look up all the original features. They offer you a price based on those features AND the CONDITION you attest to. If the condition when received is not in agreement, they will offer you less or you can cancel and have the device returned to you.