Someone said 50Kg on the 2003 Focus WRC here : http://www.motorsportforums.com/wrc/150 ... ers-6.html
That's the only number I could find, and source is unknown. I would expect the teams to put ballast on the floor of the cars of course, but also on the sump guard, since it is the lowest part of the chassis. But on the other hand, it cannot be too heavy because the mechanics have to take it off during service.