zmena inputu pomocou onchange

<form name="meno">
<select onchange="if (document.meno.size.value == '340g') { document.meno.price.value = '3.95 € / 119 Sk'; document.meno.product_id.value = '40' }if (document.meno.size.value == '630g') { document.meno.price.value = '7.27 € / 219 Sk'; document.meno.product_id.value = '164' }">
<option value="340g">340g</option>
<option value="630g">630g</option>
</select>
<input name="price" value="3.95 € / 119 Sk" readonly="readonly" type="text">
</form>

          <tr>
            <td width="20">".$value[0]."</td>
            <td width="420"><b>".$value[1]."</b><br><i>".$value[2]."</i></td>
            <td><select onchange="if (document.meno.size.value == '".$value[6]."') { document.meno.price.value = '".$value[3]."'; document.meno.product_id.value = '40' }if (document.meno.size.value == '".$value[7]."') { document.meno.price.value = '".$value[4]."'; document.meno.product_id.value = '164' }" name="size">
                <option value="".$value[6]."">".$value[6]."</option>
                <option value="".$value[7]."">".$value[7]."</option>
                </select></td>
            <td><input size="15" name="price" value="3.95 € / 119 Sk" readonly="readonly" type="text"></td>
            <td width="60" valign="top" align="center">".$value[3]." EUR</td>
            <td width="40"><input style="width: 25px;" type="text" name="produkt[".$key."]" value="0"> ks</td>
          </tr>

<form name="meno">
<?php for($a;$a<100;$a++){?>
<select name="size<?php print $a;?>" onchange="
  if (document.meno.size<?php print $a;?>.value == '340g') { document.meno.price<?php print $a;?>.value = '3.95';}
  if (document.meno.size<?php print $a;?>.value == '630g') { document.meno.price<?php print $a;?>.value = '7.27';}">
<option value="340g">340g</option>
<option value="630g">630g</option>
</select>
<input name="price<?php print $a;?>" value="3.95 " readonly="readonly" type="text">
<?php }?>
</form>