How Much Does Damage Affect Value?

I purchased a "mint" amp, cost roughly $1100. Well, packing was inadequate and the unit suffered corner damage on the faceplate during shipping. The damage is not a deal killer for me, but it is there. The unit was originally a 8/10, now I guess it would be a 6/10, though other than this blemish it is quite nice with next to no sign of wear. What would be a fair de-rate to the value?