It would also depend on which type of iPhone 4. The 4 is split into GSM an CDMA unlike the 4S and above which has a global or dual band version. GSM iPhone 4's will go for a little than CDMA 4's
I've seen people getting anywhere from 100 dollars to 200 dollars for the 16GB version. That's what they sell for in my area at least. I would personally attempt to sell it for 180 obo. That's just me though!