How to called codeigniter helper function using JQuery AJAX?

You need to make a request via controller, then call that function through that controller, something like:

$("#id").change(function()
{       
 $.ajax({
     type: "POST",
     url: base_url + "controller_name/your_function", 
     data: {val: $("#your_val").val(),currency_id: $("#your_cur").val()},
     dataType: "JSON",  
     cache:false,
     success: 
          function(data){
            $("#your_elem").val(data.price);
      }
});

Then on your controller:

public function yourfunction()
{
   $data = $this->input->post();
   $price = format_price($data['val'],$data['currency_id']);
   echo json_encode(array('price' => $price));
}