How much can I get if I trade in my iPhone?

I have an iPhone 8GB 3GS.

Not a lot, but I got more form Gamestop than Gazelle was offering.

Gazelle won't give you any money for it.
I'd recommend selling it on eBay or Crag's List.
You can sell if for 40+ on eBay.
http://www.ebay.com/...gs&_sop=12
http://www.gazelle.com/trade-in/iphone/iphone-3gs/at-t/iphone-3g-s--8gb-at-t-or-unlocked/281903-gpid#Good

My friend sold his Iphone for $175 last week.

Your iPhone is older then the dinosaurs. Roughly $35

Not much since your version is now about 5 years old. You can sell it on craigslist and there will be a buyer for it.

I don't think you'll get anything for the iPhone 3GS trade as they don't accept that anymore. But still you can try and may be you'll get $25 but that's very unlikely.

Try selling it on ebay! You still get decent prices if you put it on as an acution!