How to sum textbox value from grid display total in a label using javascript

I am having my girdivew as follows

http://www.eggheadcafe.com/FileUpload/426776068/Grid1.png

After selecting customer it will be as follows

http://www.eggheadcafe.com/FileUpload/426776068/main.png

http://www.eggheadcafe.com/FileUpload/426776068/customer.png

http://www.eggheadcafe.com/FileUpload/426776068/Selecting%20Item.png

http://www.eggheadcafe.com/FileUpload/426776068/quantity.png

http://i.stack.imgur.com/QUjzG.png

I would like to display the sum in the label available on the from

This is what I have written to display Amount in the amount textbox

<script type="text/javascript">
        function multiplication(txtQuantity, txtRate, txtAmount) {
            var weight = document.getElementById(txtQuantity).value;
            var rate = document.getElementById(txtRate).value;
            document.getElementById(txtAmount).value = weight * rate;
            var amnt = 0;
            var Grid_Table = document.getElementById('<%= grdInvoice.ClientID %>');
            for (var row = 1; row < Grid_Table.rows.length; row++) {

                var qty = 0;

                for (var col = 0; col < Grid_Table.rows[row].cells.length; col++) {

                    var cellcollectin = Grid_Table.rows[row].cells[col];

                    for (var j = 0; j < cellcollectin.childNodes.length; j++) {

                        if (cellcollectin.childNodes[j].type == "text") {
                            if (cellcollectin.childNodes[j].name == "txtAmount") {
                                alert("Success");
                                if (cellcollectin.childNodes[j].value != "") {
                                    qty = parseInt(cellcollectin.childNodes[j].value);
                                    amnt = amnt + qty;

                                }
                            }
                        }
                    }
                }
            }
            document.getElementById('<%= lblTotal.ClientID %>').innerHtml = amnt.toString();
        }
    </script>

This is how I called the script in code

for (int i = 0; i < grdInvoice.Rows.Count; i++)
                    {
                        TextBox curTexbox = grdInvoice.Rows[i].Cells[1].FindControl("txtQuantity") as TextBox;
                        TextBox curTexbox1 = grdInvoice.Rows[i].Cells[3].FindControl("txtRate") as TextBox;
                        TextBox curTexbox2 = grdInvoice.Rows[i].Cells[4].FindControl("txtAmount") as TextBox;
                        curTexbox2.Attributes.Add("readonly", "readonly");
                        Session["lbl"] = curTexbox2;

                        curTexbox2.Attributes.Add("onBlur", "return multiplication('" + curTexbox.ClientID + "','" + curTexbox1.ClientID + "','" + curTexbox2.ClientID + "')");
                        
                    }

But unable to display the total in the label can you help me