Javascript to sum drop down values

Ray - I'm close but one problem remains:

Here's my HTML:

<input type="text" id="totalcost" value="" />
<label>
<select name="prod0" id="prod0" onchange="MM_callJS('UpdateCost()')" >
<option>select</option>
<option value="100.00">one</option>
<option value="200.00">two</option>
</select>
</label>




The script:

function UpdateCost() {
var sum = 0;
var gn, elem;
for (i=0; i<29; i++) {
gn = 'prod'+i;
var dropdownIndex = document.getElementById(gn).selectedIndex;
var dropdownValue = document.getElementById(gn)[dropdownIndex].value;
{ sum += Number(dropdownValue); }
}
document.write('totalcost').value = sum.toFixed(2);
}


The values are being selected correctly - I verified that by testing a document.write statement and 'dropdown value' displays with the correct value...

The last statement in the script does not display the 'totalcost'...

Are the values not being 'summed' correctly from the drop down list?

Is the document.write statement for totalcost not correct?

Is the sum of the values not being added to totalcost?

Any help you can provide is much appreciated - this is the last item for this project I need to get working before publishing...