if i was to trade in my iphone 6s, how much would it be worth?

See the following: http://www.apple.com/iphone/trade-up/

Many cellular carriers have similar programs that may offer different prices and terms. As with all such programs you may do better from a financial standpoint by selling it yourself.

Ultimtely you will need to take it in to Apple to get a firm answer. Any estimate you get online of the trade-in device's value is conditional upon condition. So Apple needs to physically inspect the device to determine its value.

The same thing applies if you just sell it online online with someone,ace like gazelle.com. Your final value offer will be conditional upon receiving the device and inspecting its actual condition.

$225 if it is in good working condition.

http://www.apple.com/iphone/trade-up/