You need to make some changes in controller and view.
var app = angular.module('plunker', []);
 app.controller('subCCtrl', function($scope) {
   $scope.hideThisBox = false;
   $scope.update = function(label) {
     if (label == 'Cost')
       $scope.displaybox = true;
     else
       $scope.displaybox = false;
   }
 });
 app.controller('subACtrl', function($scope) {
 });
 app.controller('subBCtrl', function($scope) {
 });
HTML :
<!DOCTYPE html>
<html ng-app="plunker">
  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link rel="stylesheet" href="style.css" />
      <link rel="stylesheet" href="http://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css" />
  <script src="http://code.jquery.com/jquery-1.9.1.js"></script>
  <script src="http://code.jquery.com/ui/1.10.3/jquery-ui.js"></script>
    <script data-require="angular.js@1.0.x" src="https://ajax.googleapis.com/ajax/libs/angularjs/1.0.8/angular.min.js" data-semver="1.0.8"></script>
    <script src="app.js"></script>
  </head>
  <body>
            <div ng-controller="subCCtrl" class="row-fluid">
                <div class="span6">
                <a href="#" ng-click='update("Cost");displaybox=true;'>Cost</a>
                <br />
                <a href="#" ng-click='update("Savings");displaybox=fasle;'>Savings</a>
                <br />
                     </div>
            <hr />
            <div ng-controller="subACtrl">Do stuff that uses subACtrl</div>
            <div ng-controller="subBCtrl">Do stuff that uses subBCtrl</div>
            <hr />
            <div ng-controller="subCCtrl" class="row-fluid">
                <div class="span3">
                    <label>If click on 'Savings', hide below box: </label>
                </div>
                  <div ng-hide="hideThisBox" class="span6">
                    <input type="text" ng-model="test2" ng-show="displaybox"/>
                </div>          
            </div>
       </div>
  </body>
</html>