Adding costs in cfloop

i am trying to  create a simple invoicing script for a friend. I can get the loop to add another row and save the data. My propblem is when I try to add up the material costs. I keep getting the followinf error: Element cost1 is undefined in a Java object of type class coldfusion.filter.FormScope. (line 121)

<!--- Gets the value of getnumba from above --->

        <input type="hidden" name="numba" value="#getnumba#">

        <!--- Loop Through from 1 to the number above --->

        <cfloop from="1" to="#getnumba#" index="idx">

            <!--- At first, it displays the original one row form --->

            <!--- User enters the data and hits "Add Another" --->

            <!--- The page reloads with the data already entered and creates a new blank row that is ready for data entry --->

            <tr>

                <cfif isdefined ("form.partno#idx#")>

                    <Td><input type="text" name="partno#idx#" value="#evaluate("form.partno#idx#")#" size="62"></TD>

                <cfelse>

                    <Td><cfinput type="text" name="partno#idx#" size="62"></TD>

                </cfif>

                <cfif isdefined ("form.cost#idx#")>

                    <Td><cfinput type="text" name="cost#idx#" value="#evaluate("form.cost#idx#")#" size="6"></TD>

                <cfelse>

                    <Td><cfinput type="text" name="cost#idx#" size="6"></TD>

                </cfif>

            </tr>

        </cfloop>

<tr>

<td align="right">Materials Total </td>

<td>

   <cfset subcost = 0>

     <cfloop from="1" to="#getnumba#" index="idx">

     <cfset subcost = variables.subcost + form["cost" & idx]> (line 121)

     </cfloop>

     #subcost#

</td>

</tr>

What am I doing wrong?