Get row values and calculate in ascx Repeater

On a ascx web page I have a Repeater. The Repeater has a DDL, 3 text boxes and a "Save" button. When a user updates a number in text box "txtRate" or in text box "txtHours" in any row I want a client-side javascript function to multiply the values in those two boxes and put the result in text box "txtCost" in that same row- no postbacks involved - postback will happen when the user clicks "Save". Obviously this calculation must only apply to that particular row.
I have a javacsript function that only fires on the first row of the repeater and no other but then it applies the calculation to the "txtCost" in the SECOND row!

Function and Repeater code are attached. Any advice gratefully received.

function RPTcalculateCost()
    {
        var a = document.getElementById("ctl00$ContentPlaceHolder1$IR$ctl00$rptJobDetails$ctl01$txtRate");
        var b = document.getElementById("ctl00$ContentPlaceHolder1$IR$ctl00$rptJobDetails$ctl02$txtHours");
        var c = document.getElementById("ctl00$ContentPlaceHolder1$IR$ctl00$rptJobDetails$ctl02$txtCost");
        if (b.value == '')
        {
            c.value = 0;
        }
        else
        {
            c.value = parseFloat(a.value) * parseFloat(b.value);
        }
    }   
 
<td colspan="2" style="height: 19px">
                    <asp:Panel runat="server" ID="pnlJobDetails" DefaultButton="btnHidSave">
                        <asp:Repeater ID="rptJobDetails" runat="server">
                            <HeaderTemplate>
                            <table class="CostBox" width="100%">
                                <tr>
                                    <td align="left" style="width: 80px; height: 24px">&nbsp;&nbsp;Staff</td>
                                    <td align="left" style="width: 95px; height: 24px">Staff type</td>
                                    <td align="left" style="width: 55px; height: 24px">Hourly rate</td>
                                    <td align="left" style="width: 35px; height: 24px">Available hours</td>
                                    <td align="left" style="width: 85px; height: 24px">Cost</td>
                                    <td style="height: 24px" >&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;</td>
                                </tr>
                                <tr><td colspan="6"></td></tr>
                            </table>
                            <div style="height: 200px; overflow: auto;">
                            <table class="text" width="100%">
                            </HeaderTemplate>
                            <ItemTemplate>
                                <tr>
                                    <td>
                                        <table cellpadding="0" cellspacing="0">
                                        <tr><td>
                                        <div class="os" style="width:80px;overflow:hidden;border-right:1px solid gray">
                                        <asp:DropDownList runat="server" ID="ddlStaff" Enabled="false"  Font-Size="XX-Small" Width="180px"></asp:DropDownList>
                                        </div></td>
                                        </tr></table>
                                    </td>
                                    <td>
                                        <table cellpadding="0" cellspacing="0">
                                        <tr><td>
                                        <div class="os" style="width:80px;overflow:hidden;border-right:1px solid gray">
                                        <asp:DropDownList runat="server" ID="ddlStaffType2" Font-Size="XX-Small" Width="230px" AutoPostBack="true"></asp:DropDownList>
                                        </div></td>
                                        <td><img src="../images/arrow.jpg" /></td></tr></table>
                                    </td>
                                    <td><asp:TextBox runat="server" CssClass="costbox" ID="txtRate" Width="55px" Font-Size="XX-Small" OnKeyUp="javascript:RPTcalculateCost();" Text= <%# DataBinder.Eval(Container.DataItem, "HourlyRate")%>></asp:TextBox></td>
                                    <td><asp:TextBox runat="server" CssClass="textbox2" ID="txtHours" Width="35px" Font-Size="XX-Small" OnKeyUp="javascript:RPTcalculateCost();" Text= <%# DataBinder.Eval(Container.DataItem, "AvailableHours")%> ></asp:TextBox></td>
                                    <td><asp:TextBox runat="server" CssClass="costbox" ID="txtCost" Width="75px" Font-Size="XX-Small" Text= <%# DataBinder.Eval(Container.DataItem, "Cost")%> ></asp:TextBox></td>
                                    <td><asp:TextBox runat="server" CssClass="textbox2" ID="intCount" Visible="false" Width="1px" Font-Size="XX-Small" Text= <%# DataBinder.Eval(Container.DataItem, "interventionCount")%> ></asp:TextBox></td>
                                    <td><asp:Button runat="server" ID="btnDelete" Width="18px" Height="18px" CssClass="button" Text="X" ForeColor=red Font-Bold=true CommandArgument= <%# DataBinder.Eval(Container.DataItem, "ID") %> /></td>
                                    <td><asp:Button runat="server" ID="btnSave" Height="18px" Width="28px" CssClass="button" Text="Save" Font-Size="10px" ToolTip="Save changes" ForeColor="green" Font-Bold="true" CommandArgument= <%# DataBinder.Eval(Container.DataItem, "ID") %> CommandName="save" /></td>
                                    <td><asp:Button runat="server" ID="btnPrintDetail" Width="18px" Height="18px" CssClass="button" Text="?" ForeColor=DarkBlue Font-Bold=true ToolTip="Print details" CommandArgument= <%# DataBinder.Eval(Container.DataItem, "ID") %> CommandName="print" BackColor="LightBlue" Font-Names="Tahoma" /></td>
                                </tr> 
                            </ItemTemplate>
                            <FooterTemplate>
                                </table></div>
                            </FooterTemplate>
                        </asp:Repeater>
                    </asp:Panel>