It's exact value is whatever somebody will actually pay, and not a penny more.
Doesn't that go without saying? Is that not true for every single anything that has any monetary value?
Nothing personal petah, as everyone says it but I never understood why people say it. Whether you ask $700 for a $500 gun and hope to find the one guy who doesn't know prices who will buy, or whether you ask $250 for a $500 gun which many will people buy, the item is always worth what someone is willing to pay. Its a circular statement of sorts.
To me its akin to saying "you might have trouble shooting an unloaded firearm" - something thats true on the suface, but also doesn't help anyone with anything. The statement is considered insightful, but its not, IMO.