Add this code to an html element. Adjust the item3_text_1 to the ID of your input. Download the need JQuery currency format plugin jquery.formatCurrency-1.4.0.js and add it to your form directory. Adjust the script in the html element to point to the proper location.

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script type="text/javascript" src="../jquery.formatCurrency-1.4.0.js"></script>
<script type="text/javascript">
$(document).ready(function()
{
$('#item3_text_1').blur(function()
{
$('#item3_text_1').formatCurrency();
});
});
</script>