The total cost to rescue and then overhaul mortgage giants Fannie Mae and Freddie Mac could reach $685 billion, according to estimates published Thursday by Standard & Poor’s. Fannie and Freddie have already cost taxpayers nearly $134 billion, but S&P analysts said Thursday that the government could ultimately be forced to inject $280 billion into the firms because of a slowdown in the housing market. Any entities that might replace Fannie and Freddie would need new start-up funding that would go beyond the money already committed.