Standard rates are generally, for used systems, about 70% of average retail at whatever the LOWEST cost of those parts was during the 1-2 year period after release, plus about 10% depreciation per year of use.
So that system realistically is worth about 600-700 dollars. BUT in TRUTH, a system is only worth what you can get somebody to pay for it. If nobody is in the market or has a need for that hardware, then it's worth nothing.
If somebody has a burning need for the system, or for example, if somebody needs a Z97 motherboard but can't find one for sale anywhere and would have to fork out for a whole platform upgrade otherwise, they might be willing to pay somebody twice what the board was actually selling for new just to not have to spend more on a full upgrade.
So it's whatever the market is willing to allow. And just because it might TECHNICALLY be worth, say, 900 dollars, doesn't mean you will EVER find somebody worth spending that much on it because chances are good anybody willing to spend an amount over 500 bucks is probably also willing to spend a few more bucks to make it worth it on whatever the current generation platform is as well.