Mint or close to it iPhone 4S's have dropped in value in the last few weeks.
I think it's a terrific iPhone and was surprised it hasn't held its value longer.
With that stated, iOS8 models tend to garner $75.'ish and iOS7's perhaps $15. to $20. more.
It's the iOS6 configurations that approach $125. or a bit more depending on 16GB; 32GB, or 64GB.