For American taxpayers, now on the hook for some $145 billion in housing losses connected to Fannie Mae and Freddie Mac loans, that amount could be just the tip of the iceberg. According to the Congressional Budget Office, the losses could balloon to $400 billion. And if housing prices fall further, some experts caution, the cost to the taxpayer could hit as much as $1 trillion. Two things are clear: Taxpayers don’t want to foot the bill, and Fannie and Freddie, taken over by the government in 2008 to stanch the financial bloodletting, need a major overhaul.