If they have even wear, and are in decent shape (not weather checked), I think half the price of new is fair, and probably my max.
At 8K miles of wear, they are still about 75% give or take, so discount 25% at least over the use value. Remember, you are saving tax too vs new (if you have local taxes) but mounting/balancing will be added on.
And FWIW, I have the KM2's; consider having them siped before you install them. It really helps on snow/ice/wet roads, and smooths them out as well
"...but in dog beers, I've only had one..."