"The principle of revealed preference." If he's not buying it, he's not really valuing it. Like the old joke about two economists who walk past a porsche. One remarks that he'd give anything to own that car --- the other responds, "Clearly not."
An artwork is not a porsche. Even if I had anywhere near enough money to buy a Kiefer, there would be no room for it in my house, and it would not belong there anyway. I think it is valuable simply that Kiefer's paintings exist, and would be even if museum's weren't willing to pay for them.
Exactly!