I think it is just what the market will pay for a used shadow target.
A few months ago I found one at a small gun shop up north, a shop which seems to get a lot of used cz and sti guns. I think there is a healthy amount of guys with higher end guns in that region. The shop was asking $900 for the shadow target I picked up. It is very lightly used, almost new condition. This was about 25% lower than the new price.
I think this 25% price reduction for a used gun can be seen across other platforms. I have an almost new m&p that would have been about $400 new. Nobody wants to pay more than $300 on it, which is right at 25% off new price.