The answer is that the value varies a great deal depending on exact version of the gun, where you are in the USA, the gun's condition, and current market for that model.
The online auction sites and their completed sales are probably the best indication of value. You must adjust for condition, and where you are trying to sell it.
Clearly, a forum like this can't give accurate estimates without knowing details. Gun shops have to make a profit, and they'll offer something different that a private buyer would. They'll also sell it at a markup for profit, establishing an approximate insurance value for replacing one.