It depends on many factors. What season is it from? What color is it? How "used" is used?
You got to it first Melly! That's exactly what I was about to say.
I'd say a Fall 05 bag could fetch a lot, esp if it's a highly desired color like violet or petrol, possibly more than the retail value. The other 05 colors or stams with suede interiors would probably get near retail if you could prove beyond a doubt that the bag was authentic and it was in extremely good condition. If the bag is used and near mint, you'll probably get btw $800-600 dollars.
The canvas lined bags still do really well, I'd say around $800 for a BNWT in excellent condition.