it's true that copies sometimes go for $300, but that's rather exceptional. i wouldn't count on that, even if it's signed.
$100-120 is a more realistic expectation for an unsigned copy. i'm not sure how much value the signature will add. could be a lot, could be very little.
if your friend is expecting $400+, he's very likely to end up disappointed. i'm not saying it's impossible, but you need to be very lucky.