title: Jquery: Change existing price with checkbox value
body: <p>I have a checkout form where the base price is $249.99. The user has the option to add a desk and or chair for $99.99. As of right now the price changes, but removes the base price instead of adding to it. And upon de-selection, it actually subtracts the base price leaving a total of $0.</p>
  
  <pre><code>$(':checkbox').change(function(){
      var sum = 0;
      var names = $(':checked').map(function(){
          sum += (this.value - 0);
          return this.name;
      }).get().join(',');
      var spans = $('span.amount');
      spans[1].innerHTML = sum;
  });
  </code></pre>
  
  <p>I've made a <a href="http://jsfiddle.net/XSCZ3/6/" rel="nofollow">fiddle here</a> to show what I mean. So how do I get the base price to stay $249.99 and do the proper math? Thanks so much for your help in advance.</p>