"There is a lack of a fundamental pricing model for art. Aside from the psychic pleasure art produces, there is no clear income stream that flows from art. Other assets have both an income stream as well as a potential capital gain. Stocks provide dividends, bonds provide coupon payments, and real estate can generate rent. Art, in some ways, is closer to gold. But at the same time art is less liquid and less homogenous than gold. Copper is copper. Gold is gold. A share of Apple or GM is always the same. Meanwhile, not only is one piece of art not the same as another, but there are even different measuring standards for the same artist depending on the medium - drawings versus paintings, for example - and at what point in the artist's career the work was created."
Nouriel Roubini in Why Art Is an Asset Class