bought my '05 6MT sedan for $19,000 back in Sept last year, with 74,000km on it. It's a non-premium, but has 18in coupe rims and a few optional extras.
From what I have seen, the market has dropped off quite a bit since then, and I think I would only get around $17k at the most if I sold it now. so an '03 would probably be closer to $15k - $16k.
However, i do agree with n15m0, not everyone prefers a coupe. I was looking for a Sedan and wouldn't have been interested in a coupe even if I could get one with the same specs/age for the same price. That said.. coupes generally do sell for more.
If there are no other 350gt sedans on carsales for buyers to compare with, you might get lucky by listing it at $20k.. although the average V35 buyer has probably done their homework.