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Provided your iPhone is still in good condition and especially if it still has an unlockable modem firmware version on it, then it will fetch a good amount. Why not search ebay yourself to see what they're going for right now, hmmm?
 
Why not do a search for completed listings? You'll get better answers than from the tiny sample of MR users.

I'm selling my 32gb white to a friend for $425.

Man, charging your friend $425 for a 32gb white iPhone 3GS? Some friend ;)
 
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