Edit in JSFiddle

//Demo by Brij Mohan (http://techbrij.com) 
//Qty Validation and Total Calculation in Gridview Shopping Cart with jQuery

$(function () {           
          

            //Change price and grand total on changing qty
            $('#GridView1 .txtQty').blur(function () {
                var $tr = $(this).parents('tr');
                if ($tr.length > 0) {
                    if (parseInt($tr.find('.avlQty').html()) < $(this).val()) {
                        alert('Qty must not exceed available quantity.');
                        var $ctrl = $(this);
                        window.setTimeout(function () {
                            $ctrl.focus();
                         }, 50);
                        return false;
                    }
                    $tr.find('.totalPrice').html(parseFloat($tr.find('.price').html()) * parseInt($(this).val()));
                }
                CalculateGrandTotal();
            });
            //To get grand Total
            function CalculateGrandTotal() {
                var grandTotal = 0.0;
                $('#GridView1 tr:gt(0)').each(function () {
                    grandTotal = grandTotal + parseFloat($(this).find('.totalPrice').length == 0 || !$(this).find('.totalPrice').html() ? 0 : $(this).find('.totalPrice').html());
                });
                $('#GridView1 .grandtotal').html(grandTotal);
            }
            //First Time to display all total amount and grand total
            function initGrid() {
                $('#GridView1 tr:gt(0)').each(function () {
                    $(this).find('.totalPrice').html(parseFloat($(this).find('.price').html()) * parseInt($(this).find('.txtQty').val()));

                });
                CalculateGrandTotal();
            }
            //To allow numeric character only
            $('#GridView1 .txtQty').keydown(function (event) {
                // Allow: backspace, delete, tab, escape, and enter
                if (event.keyCode == 46 || event.keyCode == 8 || event.keyCode == 9 || event.keyCode == 27 || event.keyCode == 13 ||
                // Allow: Ctrl+A
            (event.keyCode == 65 && event.ctrlKey === true) ||
                // Allow: home, end, left, right
            (event.keyCode >= 35 && event.keyCode <= 39)) {
                    // let it happen, don't do anything
                    return;
                }
                else {
                    // Ensure that it is a number and stop the keypress
                    if (event.shiftKey || (event.keyCode < 48 || event.keyCode > 57) && (event.keyCode < 96 || event.keyCode > 105)) {
                        event.preventDefault();
                    }
                }
            });

            initGrid();
        })

<table cellspacing="0" rules="all" border="1" id="GridView1" style="border-collapse:collapse;">
        <tr>
            <th scope="col">Code</th><th scope="col">Name</th><th scope="col">Available Qty</th><th scope="col">Price</th><th scope="col">Qty</th><th scope="col">Net Price</th>
        </tr><tr>
            <td>ASD123</td><td>Product 1</td><td class="avlQty">10</td><td class="price">12.50</td><td>                
                    <input name="GridView1$ctl02$TextBoxQty" type="text" value="1" maxlength="5" id="GridView1_TextBoxQty_0" class="txtQty" style="width:45px;" />
                    <span id="GridView1_RequiredFieldValidator1_0" style="color:Red;display:none;">*</span>
                </td><td>
                    <span class="totalPrice"></span> 
                </td>
        </tr><tr>
            <td>BBB123</td><td>Product 2</td><td class="avlQty">12</td><td class="price">15.00</td><td>                
                    <input name="GridView1$ctl03$TextBoxQty" type="text" value="1" maxlength="5" id="GridView1_TextBoxQty_1" class="txtQty" style="width:45px;" />
                    <span id="GridView1_RequiredFieldValidator1_1" style="color:Red;display:none;">*</span>
                </td><td>
                    <span class="totalPrice"></span> 
                </td>
        </tr><tr>
            <td>CCC123</td><td>Product 3</td><td class="avlQty">15</td><td class="price">20.00</td><td>                
                    <input name="GridView1$ctl04$TextBoxQty" type="text" value="1" maxlength="5" id="GridView1_TextBoxQty_2" class="txtQty" style="width:45px;" />
                    <span id="GridView1_RequiredFieldValidator1_2" style="color:Red;display:none;">*</span>
                </td><td>
                    <span class="totalPrice"></span> 
                </td>
        </tr><tr>
            <td>DDD123</td><td>Product 4</td><td class="avlQty">20</td><td class="price">37.50</td><td>                
                    <input name="GridView1$ctl05$TextBoxQty" type="text" value="1" maxlength="5" id="GridView1_TextBoxQty_3" class="txtQty" style="width:45px;" />
                    <span id="GridView1_RequiredFieldValidator1_3" style="color:Red;display:none;">*</span>
                </td><td>
                    <span class="totalPrice"></span> 
                </td>
        </tr><tr>
            <td>EEE123</td><td>Product 5</td><td class="avlQty">21</td><td class="price">25.00</td><td>                
                    <input name="GridView1$ctl06$TextBoxQty" type="text" value="1" maxlength="5" id="GridView1_TextBoxQty_4" class="txtQty" style="width:45px;" />
                    <span id="GridView1_RequiredFieldValidator1_4" style="color:Red;display:none;">*</span>
                </td><td>
                    <span class="totalPrice"></span> 
                </td>
        </tr><tr>
            <td>&nbsp;</td><td>&nbsp;</td><td>&nbsp;</td><td>&nbsp;</td><td>&nbsp;</td><td>
                   Total: <span class="grandtotal"></span> 
                </td>
        </tr>
    </table>

table tr td
    {
        padding:5px 3px;
    }
     table tr td input
    {
        border:solid 1px gray;
    }