<script type="text/javascript">
    function Points(pln) {
        if(is_numeric(pln)) {
            document.getElementById('pkt').innerHTML = Math.floor(pln*200)+' pkt';
        } else {
            document.getElementById('pkt').innerHTML = '0 pkt';
        }
    }
</script>


                kwota: <input type="text" name="kwota" id="pln" size="15" onChange="Points(this.value)" /> PLN<br />

kwota: <input type="text" name="kwota" size="15" onChange="if(!isNaN(this.value)) {document.getElementById('pkt').innerHTML = Math.floor(this.value*200)+' pkt';} else {document.getElementById('pkt').innerHTML = '0 pkt';}" /> PLN<br />

<span id="pkt">0 pkt</span><br />

kwota: <input type="text" name="kwota" size="15" onChange="document.getElementById('pkt').innerHTML = (parseFloat(this.value)*200)||0+' pkt'" /> PLN<br />

<span id="pkt">0 pkt</span><br />