Brug en SELECT box onChange-rutine til flere Select-bokse..

<tr><td>billede</td><td>titel</td><td><select onchange="changePrice('1', this, '14,95');" name="amount_1" id="amount_1"><option value="1" selected>1</option><option value="2">2</option><option value="3">3</option></td><td><input type="readonly" name="price_1" id="price_1" value="14,95"></td></tr>
<tr><td>billede</td><td>titel</td><td><select onchange="changePrice('2', this, '399,95');" name="amount_2" id="amount_2"><option value="1" selected>1</option><option value="2">2</option><option value="3">3</option></td><td><input type="readonly" name="price_2" id="price_2" value="399,95"></td></tr>
<tr><td>billede</td><td>titel</td><td><select onchange="changePrice('3', this, '749,50');" name="amount_3" id="amount_3"><option value="1" selected>1</option><option value="2">2</option><option value="3">3</option></td><td><input type="readonly" name="price_3" id="price_3" value="749,50"></td></tr>

<script type="text/javascript">
   function changePrice(intLineId, objSelect, fltPrice){
      var objPriceField = document.getElementById('price_'+intLineId);
      var intAmount = parseFloat(objSelect.value);
      var fltTotalPrice = parseFloat(fltPrice.replace(',', '.') * intAmount);
      var strTotalPrice = fltTotalPrice.toFixed(2)+'';
      strTotalPrice.replace('.', '.');
      objPriceField.value = strTotalPrice;
   }
</script>