There's a couple of methods you can use to do this. The first isn't the most practical but it's a good starter. The second is a much more complicated process simplified.
1. Go to eBay and price out the parts individually and you can see what they're worth and add them all up. That will be one indicator.
2. With depreciation it takes about 1/2 of what your system is worth, and that would be a selling point.
3. You might have better luck stripping the rig and selling the parts individually. I've had better luck with that than I have selling full rigs.