This is all relative, I guess. If the bag is NWT then $1500 isn't totally out of the ball park price-wise. Since G21 isn't made anymore and most of them have been used, they can command higher prices. I've seen several new or like new black G21 City bags listed in this range, but they typically don't sell very quickly (or at all until the price drops a bit) which tells me that it's probably a bit above market. That said, if you want NWT and 2009 is your preferred year, then perhaps this is a worthwhile price for you (with whatever negotiation the seller will accept).
The years 2007, 2009 and 2011 all had reputations for the black fading to green, so I'd just be sure that this isn't the case with this bag if you do pay the premium for it. On the other hand, 2009 also had some really gorgeous, soft, beautifully smooth leather that was unlike other years, and therefore it has it's own admirers and followers as well.
Every bag is unique... hopefully the seller is willing to answer your questions and provide as many photos as you need to feel comfortable with your decision, especially if they don't offer return/refund. If you're fortunate enough to be talking to a seller that DOES offer returns, then the situation is very low risk and you can buy it, check it out, and then send it back if it's not all you dreamed of!
I believe there is a black 2011 G21 GGH pre-loved in excellent condition from a canadian seller on eBay right now in the (don't quote me) $1300-$1400 range, and it may have a best offer on it, too. I'd say that unless you are absolutely insistent upon 2009 and NWT, you will have other options at lower price points (if price matters to you, for some people it doesn't) for bags from other years in nice condition. But, again, so much goes into these decisions and it's so personal, it's hard to say where "value" really lies. I would just be very sure the bag is in absolute top notch condition and with leather you will like before you pull the trigger!
Sorry to be so lengthy, but I do hope it helps!