AngularJS Example:

by luizz

HTML

<div ng-app>
  <div>
    <label>Desp. Fixas Mensal :</label>
    <input type="text" ng-model="desFixaMensal" ng-change="salTres={desFixaMensal/12}"  placeholder="R$">
    <label>13º Sal.:</label>
    <input type="text" value="{{salTres}}" placeholder="R$">    
    <label>Lucro :</label>
    <input type="text" ng-model="lucro" placeholder="%">
    <label>Num de Horas :</label>
    <input type="text" ng-model="nHoras" placeholder="Horas">
    <hr>
        <h2>Total: R$ {{desFixaMensal+lucro}}</h2>
  </div>
</div>

CSS

</style> <!-- Ugly Hack due to jsFiddle issue: http://goo.gl/BUfGZ --> 
<link rel="stylesheet" href="http://twitter.github.com/bootstrap/assets/css/bootstrap.css">
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.0.5/angular.min.js"></script>
<style>

JavaScript

function calc($scope) {
    
}