invoking keyup for javascript method in dynamic created textbox with dynamic name

i used this code to create textboxes

            txtCost = new TextBox();
            txtCost.ID = "txt" + dtjCostCenter.Rows[i]["int_CostCenterId"].ToString();
            txtCost.MaxLength =12;
            //txtCost.Text = dtjCostCenter.Rows[i]["int_CostCenterName"].ToString();
            phcosts.Controls.Add(new LiteralControl(""));
            phcosts.Controls.Add(txtCost);
            AjaxControlToolkit.FilteredTextBoxExtender ftbx = new AjaxControlToolkit.FilteredTextBoxExtender();
            ftbx.ID="ftb" + dtjCostCenter.Rows[i]["int_CostCenterId"].ToString();
            ftbx.ValidChars = "1234567890.";
            ftbx.TargetControlID = "txt" + dtjCostCenter.Rows[i]["int_CostCenterId"].ToString();
            ftbx.FilterType = AjaxControlToolkit.FilterTypes.



about 5 textboxes are created . now the thing is that i want to calculate auto sum of every textbox on keyup event and place that in total label. i have performed this thing with the non dynamic textboxes with non dynamic name
like this with javascript function being called on keyup

script language="javascript">
    function sum() {
        var mySum = 0;
        var value1 = parseInt(document.getElementById("TextBox1").value);
        var value2 = parseInt(document.getElementById("TextBox2").value);
        var value3 = parseInt(document.getElementById("TextBox3").value);

        if (value1 > 0) {
            mySum = mySum + value1
        }
        if (value2 > 0) {
            mySum = mySum + value2
        }
        if (value3 > 0) {
            mySum = mySum + value3
        }
        document.getElementById("TextBox4").value = mySum
    }

now how would i invoke keyup in my c# code to invoke the sum method.