Try like this..
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<script src="jquery.js.js"></script>
<script type="text/javascript">
var calc = function (txt) {
var rowid = txt.id.split('_')[2];
var Rate = parseFloat(document.getElementById('GridView1_txtRate_' + rowid).value);
var Qty = parseFloat(document.getElementById('GridView1_txtQty_' + rowid).value);
document.getElementById('GridView1_txtAmt_' + rowid).value = Rate * Qty;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField HeaderText="Rate">
<ItemTemplate>
<asp:TextBox ID="txtRate" onblur="calc(this);" runat="server"></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Qty">
<ItemTemplate>
<asp:TextBox ID="txtQty" onblur="calc(this);" runat="server"></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Amt">
<ItemTemplate>
<asp:TextBox ID="txtAmt" runat="server"></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</form>
</body>
</html>