This week, the U.S. Supreme Court considers whether to count self-identified illegal immigrants in the 2020 census. Cities worry adding the citizenship question could undercount 6.5 million people. Their argument, however, isn't just about political power but billions of dollars in federal funds states expect. The case underscores what experts say is a growing cost to taxpayers from the surge of Central American families and unaccompanied minors. "We're talking about billions of dollars in taxpayer benefits over the next few years," said Dan Stein, director of the right-leaning think tank, Federation for American Immigration Reform. "The payout for the taxpayer...