What's a good price for a carrier locked 16GB iPhone 4S or 5S?

The carrier is AT&T. I'm asking because I don't want to get ripped-off with a price that isn't reasonable.

The fact that the device wold be "unlocked' means it would be preowned. So the price range is about $100-$150. Of course there will be people who try to sell it to you for more, but that's not worth it for that model of phone. If you're going to pay more than $150, you might as well just buy the 16GB iPhone 6 brand new.