The value of art

There has been much press about Damien Hirst's $100m skull. The diamonds cost $24m so his return on investment is just over 4 times, critics say, which is nothing compared to the ROI on Van Gogh's portrait of Dr Gachet where the costs were paint and canvas and which sold for $117m in 1990 (ok, it wasn't in his lifetime).
Do you remember the story of Saatchi buying the head made of frozen blood for his new wife, Nigella Lawson? He thought it was be a good joke not to tell her about it and to put it in the freezer in their kitchen so that she'd have a shock when she came to prepare dinner. However, the builders were working on their house and they switched off the electricity so Nigella came home to find a stream of blood emanating from the freezer onto the kitchen floor. Rather a waste of £25,000.