title: Simple Percentage Calculation of an Input Field with JQuery
body: <p>I'm building a PayPal form that the user enters the amount and after that transfered to paypal,<br>
  everything is working fine but i need to add a calculation plugin to do the following,  </p>
  
  <p>If a user enters in the field 10$ (Calculate 10$-2%=8$) and return the result as text inside a span 8$  </p>
  
  <p>At the end of the form i have a "(p) paragraph" with a span id "site_commission_box" inside.<br>
  I need to show the calculated number inside this span.  </p>
  
  <p>Which JQuery plugin is good for use on this and how i can use it?<br>
  Any good example or tutorial to find out how i can do it?  </p>
  
  <p>Thanks a lot,<br>
  Philip</p>
  
  <pre><code>&lt;form id="form_paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post"&gt;
      &lt;input type="hidden" name="business" value="mail@url.url"/&gt;
      &lt;input type="hidden" name="cmd" value="_xclick"/&gt;
      &lt;input type="hidden" name="return" value="http://returnpage.url"/&gt;
      &lt;input type="hidden" name="cancel_return" value="http://cancelreturnpage.url"/&gt;
      &lt;input type="hidden" name="rm" value="2"/&gt;
      &lt;input type="hidden" name="currency_code" value="USD"/&gt;
      &lt;input type="hidden" name="quantity" value="1"/&gt;
      &lt;input type="hidden" name="item_name" value="Item Name"/&gt;
      &lt;input type="hidden" name="item_number" value="Item ID"/&gt;
  
      &lt;label&gt;Amount (US$) : &lt;/label&gt;
      &lt;input type="text" name="amount" id="input_amount" class="text" /&gt;
  
      &lt;input type="hidden" name="invoice" value="Post ID"/&gt;
      &lt;input type="hidden" name="cbt" value="Make Payment &amp;rarr;"/&gt;
      &lt;input type="hidden" name="item_number" value="Item ID"/&gt;
      &lt;input type="submit" name="submit" value="Confirm &amp; Pay &amp;rarr;" class="submit" /&gt;
      &lt;br/&gt;
      &lt;span id="msg_moreamount" class="icon_warning red" style="display:none;"&gt;PayPal takes $0.35 commission for a $1 donation.&lt;/span&gt;
      &lt;span id="msg_noamount" class="icon_warning red" style="display:none;"&gt;Please enter an amount and try again.&lt;/span&gt;
      &lt;span id="msg_activity" style="display:none;"&gt; &lt;img src="loader.gif" align="middle" alt="load"/&gt;&amp;nbsp;Transferring to PayPal, please wait...&lt;/span&gt;
  
      &lt;p&gt;-2% of the price it will be &lt;b&gt;$&lt;span class="text_decoration" id="site_commission_box"&gt;&lt;/span&gt; USD&lt;/b&gt;.&lt;/p&gt;
  &lt;/form&gt;
  </code></pre>