One of the most common questions regarding any "vintage" piece of gear is: "How much is it worth?" There are only TWO correct answers. The FIRST one is: "Whatever you can get for it." The second one is: "What is it worth to YOU?"
These are not "flip" answers. There is a very limited market for "vintage" audio gear and SO much is dependent upon condition that it is completely IMPOSSIBLE for someone to answer your question as stated. If a unit is "totally stock and un-touched," it might fetch more OR less, depending upon a potential buyer's wants or needs. If it has been completely rebuilt with new parts, it could be worth a good deal, OR next to nothing - again, depending upon a buyer's needs or wants.
For the BEST "take" on the value of an HK-250, I would contact Jim McShane. He specializes in Harmon-Kardon "Citation" units. But if ANYONE knows the value of an H-K unit, I'd bank on Jim. You can e-mail him at:
jimmcshane(at)prodigy.net
I DO hope this helps.
Bill
P.S. I have attached a copy of the HK-250 Owner's Manual in pdf form - just in case you don't have it.
Bill