javascript - Unable to get textfield value using jQuery

Question

I have an invoice.jsp page where I have to calculate some value in the textbox using jQuery.

In my invoice there is a quantity textbox. If the user enters the quantity then the calculated price should be calculated dynamically i.e (total_subPrice= unit_price * quantity) and shown in another textbox called "price".

Now my current output is like this:

I tried the code below to solve this issue, but my jQuery code is unable to get the unitprice textfield data. I don't know why. Please check my code below and suggest me a solution for it.

invoice.jsp

--------------------
--------------------
<script type="text/javascript">
  $(document).ready(function(){
    $(function() {
      $('input[name^="quantity"]').change(function() {
        var unitprice = $(this).siblings('input[name^="unitprice"]').val();
        alert("Unit price check="+unitprice);
        //problem in this line. Unable to get the unit price
        $(this).siblings('input[name^="price"]').val($(this).val() * unitprice);
      });
    });
  });
</script>
..............................
..............................
<!--
  Here I am iterating through a list from my dabase using strus2 framework tags.
  And defined my  input field values in struts2 tag eg. `<s:textfield.../>`
  is the same as `<input type="text".../>
-->
<table width="100%" height="100%" border="0" cellpadding="0" cellspacing="0">
  <s:iterator  value="#session.BOK" status="userStatus">
    <tr style="height: 10px;">
      <td width="65%" align="left"><s:property value="bookTitile"/></td>
      <td width="10%" align="left">
        <s:textfield name="unitprice" value="%{price}" size="4"/>
      </td>
      <td width="10%" align="center">
        <s:textfield name="quantity" value="%{quantity}" size="2" />
      </td>
      <td width="15%" align="center">
        <s:textfield value="%{price}" name="" size="6"></s:textfield>
      </td>
    </tr>
  </s:iterator>
</table>
................................
................................

When I change any value in my quantity textbox, my alert box is showing "Unit price check=undefined". Please check my code and suggest any solution.

Update: Generated html

 inside loop {
  <tr style="height: 10px;">
    <td width="65%" align="left">book title display</td>
    <td width="10%" align="left">
      <input type="text" name="unitprice" value="%{price}" size="4"/>
    </td>
    <td width="10%" align="center">
      <input type="text" name="quantity" value="%{quantity}" size="2" />
    </td>
    <td width="15%" align="center">
      <input type="text" value="%{price}" name="" size="6"></s:textfield>
    </td>
  </tr>

Answer 1

I got it working: http://jsbin.com/efinak/2/edit

$(function() {
  $('input[name="quantity"]').change(function() {
      var unitprice = $(this).parents('tr').find('input[name="unitprice"]').val();
      $(this).parents('tr').find(' input[name="price"]').val($(this).val() * unitprice);

    });
});