If you're using the NAB module from the downloads area, it should already be unformatted.

ie: lines 80-92:
Code:
    function process_button() {
	    global $_POST;  global $db, $order_totals, $order;
//	  $total=$order->info['order_totals'];             
      $amt=$order->info['total'];             
// Calculate the next expected order id
    $last_order_id = $db->Execute("select * from " . TABLE_ORDERS . " order by orders_id desc limit 1");
    $new_order_id = $last_order_id->fields['orders_id'];
    $new_order_id = ($new_order_id + 1);
	
     $process_button_string = zen_draw_hidden_field('merchid', 'B5768139B28F28B5D760B7DC415447500B66639247B568983D48349F5662C392') .
//      zen_draw_hidden_field('mtid', $new_order_id) . 
	zen_draw_hidden_field('mtid', '10003') . 
	zen_draw_hidden_field('amt', $amt) .
$order->info['total'] is already an unformatted value