sommare valori contenuti nei listbox

[polpone]

Guarda questa foto, forse ti sarà piu chiaro.

Metto anche il codice:
<?php
echo "<select name=\"select\">
<option value=\"123\">qwe - 123&euro;</option>
<option value=\"456\">asd - 456&euro;</option>
<option value=\"789\">zxc - 789&euro;</option>
</select>
name 1";

echo "<select name=\"select2\">
<option value=\"111\">qqq - 111&euro;</option>
<option value=\"222\">www - 222&euro;</option>
<option value=\"333\">eee - 333&euro;</option>
</select>name 2";

echo "<select name=\"select3\"><option value=\"100\">prezzo base - 100&euro;
</option></select>prezzo base";


echo "<select name=\"select4\"><option value=\"$tot\"></option> </select> prezzo finale";

?>