I don't know about the raw numbers, but the folks at the local GM will tell you what they can offer, and why they can offer that much(Condition, why they rated it that condition, resale price, their profit). I've heard them quite often say that a seller could get more if they tried to sell privately, at a show, etc.
*edit* Even if you don't sell/trade with them, its a decent indicator of potential sale value, and local market. Sometimes, local markets just don't support certain firearms, so what you could get elsewhere, is not what you could get locally.