Obviously the Mafia doesn't release quarterly statements of it's finances, so there's really no way to know something like that. The Italian government and various "think tanks" have estimated the Mafia in Italy's worth at anywhere from 7-10% of the nation's GDP or 15% of it's GNP. In recent years we've seen figures like $167 billion earned by the Italian Mafia groups annually. I don't know how much stock I put in estimates like that though.
But, for what it's worth, check out this article. It gives a breakdown of supposed Mafia worth -http://news.bbc.co.uk/2/hi/europe/1023221.stm