In this article, Kurt Kasun reproduces a chart from Marc Faber's latest newsletter, showing an estimated drop of c. 50% on the world's stockmarkets - a loss of some $30 trillion.
So I've looked at the Dow and the FTSE, as priced in Euros, since the Euro appears to be more stable than either the dollar or the pound sterling (until we discover the supermassive black hole at the centre of the European financial galaxy, no doubt).