As with anything used it is worth exactly what one other person is willing to buy it for. Condition, finish and location play a huge role. Beat up raw birch that need new crossovers and require 10 hour drive each way for $2000 is probably not a smart use of your time and money in my book, but if you have to have them because you have every other type of wood finish and simply must complete your collection of 25 pairs of Khorns I'd say have at it. Likewise, I'd say if there was a pair of Teak Khorns the next city over for $3,500 and I really wanted them I'd be off with money in hand and hope to resell my OO or WO more common pair to offset the premium for the Teak. Zebrawood in perfect condition would be worth a ton to me, probably not everyone else. Some people might really want a black ash pair because they don't care about looks, sound only or they fit perfectly into their décor. I'd not take them for $1,000 if the stipulation was I had to live with them the rest of my life. I'd rather listen to Forte IIs or Cornwalls that looks nice over anything black in my particular house....but that is just me.
Now factor in the enjoyment you get out of them. Is waiting 18 months to get a pair for $2,000 vs $2,500 really going to change your life? That is 18 months at easily $50 a month enjoyment (my personal enjoyment value). So you saved 500 bucks but lost 900 in enjoyment. Well done Mr. Rockefeller! For the most part I've found the "average" Klipsch forum member to be a tight a$$ when it comes to $$$. Where did all these used Khorns come from? Somebody bought them day 1 new and thought they were worth MSRP or much closer to that than the typical used selling price, or bragged about buying price on this forum. I don't think we are the norm here. We obsess over this XO or that phase plug. Really, is a soldered midrange driver going to put you into audio bliss and an AK-2 going to make you vomit every time you drop the needle?
All that said, if you are talking common OO or WO, nothing to special and they would look good to anyone casually passing through you living room I think $2,500 +/- is a good starting point. Up for upgrades that matter to you, down for condition or black/raw. Exotic woods or they just speak to you in some way so you gotta have them...get out your wallet. Nice to have but you really don't need them....throw out a lower offer and see what happens.