How do you value art?

The art world is abuzz with the story of Bansky’s latest prank- a painting of his sold for £1m in Sothebys and it began to self-destruct as soon as the hammer fell on the sale.
 
 
It raises the old question: how do you value art?  Some critics now say the painting , half in shreds , is worth at least 50 per cent more…
 
 
Meanwhile here is a simple black and white photo of some sea holly in the garden.