Assuming you would like an answer for how much an iPhone 6 would sell for today, the average price for an iPhone 6 is about $220. This number can change based on many different factors such as whether the phone is locked to a carrier, the condition of the device, and which storage capacity you have (16GB, 32GB, 64GB, 128GB).
The best way to find out how much your specific device is worth is by checking online resale sites or taking it into a cell phone store.