Try this
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<head runat="server">
<script src="../Script/jquery-1.6.4.js" type="text/javascript"></script>
<title>Test Demo</title>
<script type="text/javascript">
function CheckNumber(e)
{
var charCode = (e.which) ? e.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return false;
return true;
}
function AddValue(e)
{
//Require jquery
var Total=0;
for(var i=1;0<$("#<%=Gridview1.ClientID%> tr").length-1;i++)
{
if($("#<%=Gridview1.ClientID%> tr:eq("+i+") input:[id*='TextBox1']").val()!="")
{
Total = Total + parseInt($("#<%=Gridview1.ClientID%> tr:eq("+ i.toString() +") input:[id*='TextBox1']").val());
}
if(($("#<%=Gridview1.ClientID%> tr").length-2)==i)
break;
}
$("#<%=txt.ClientID%>").val(Total);
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:GridView ID="Gridview1" runat="server" ShowFooter="true" CssClass="vutblrow"
TabIndex="3" HeaderStyle-CssClass="vutblhdr" CellPadding="4" ForeColor="#333333"
GridLines="None" Width="25%" PagerStyle-Mode="NumericPages" AutoGenerateColumns="false"
Height="16px" OnRowCreated="Gridview1_RowCreated">
<PagerStyle CssClass="pgr" Height="25px" BorderStyle="Solid" />
<Columns>
<%--<asp:BoundField DataField="RowNumber" HeaderText="Serial Number" />--%>
<asp:TemplateField HeaderText="Serial Number">
<ItemTemplate>
<%# Container.DataItemIndex+1 %>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Air/Bus/Train Fare">
<ItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" onkeypress="return CheckNumber(event)" onkeyup="AddValue(this)" CssClass="txtBoxNormalmedium"></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="City To">
<ItemTemplate>
<asp:TextBox ID="TextBox2" runat="server" CssClass="txtBoxNormalmedium"></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="City From">
<ItemTemplate>
<asp:TextBox ID="TextBox3" runat="server" CssClass="txtBoxNormalmedium"></asp:TextBox>
</ItemTemplate>
<FooterStyle HorizontalAlign="Right" />
<FooterTemplate>
<asp:Button ID="ButtonAdd" runat="server" Text="Add New Row" OnClick="ButtonAdd_Click"
CssClass="btnNormalAdd" />
</FooterTemplate>
</asp:TemplateField>
<asp:TemplateField>
<ItemTemplate>
<asp:LinkButton ID="LinkButton1" runat="server" OnClick="LinkButton1_Click" CssClass="lnkbut">Remove</asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
<asp:Label ID="lblgpf14" runat="server" CssClass="lblText " Text="Total Air Fare"></asp:Label>
<asp:TextBox ID="txt" runat="server" CssClass="txtBoxNormalmedium" Enabled="False"></asp:TextBox>
</div>
</form>
</body>
</html>