How much is an iPhone 4 in fair-good condition worth?

I just ordered an iPhone 5s and am not sure if I should keep my iPhone 4 as a back up, or try to sell it. It's either 8GB or 16GB, not sure at the moment.
The exterior is in almost like new condition very slight wear in a couple spots. And everything is fully functional as you would expect except the top button doesn't work (lock button I think it's called).
So given this info, would you knowledgable folks say the phone is worth at least $50? If not I'll be keeping it

Well as in stores today iphone 4 are free you don't even have to pay for them

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