codice:
<input type="text" name="km[<?php echo $i; ?>]" value="1" onchange="somma(<?php echo $i; ?>)"/>
<input type="text" name="prezzi_u[<?php echo $i; ?>]" value="<?php echo $listino[$i]['prezzo']; ?>" onchange="somma(<?php echo $i; ?>)"/>

<input type="text" name="prezzi_t[<?php echo $i; ?>]" value="1"/>
<script>
function somma(index)  {
  var field1 = document.form_name.elements["km" + index];
  var field2 = document.form_name.elements["prezzi_u" + index];
  var field3 = document.form_name.elements["prezzi_t" + index];
  field3.value = parseFloat(field1.value) + parseFloat(field2.value);
}
</script>