<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<body>
<input class="fiyat">
<input class="kdv">
<input class="toplam" disabled="" id="sayi2">
</body>
<script>
$('input.fiyat,input.kdv').on('change keyup', function(){
var $fiyat = $('input.fiyat'),
$kdv = $('input.kdv'),
$total = $('#sayi2');
$total.val($fiyat.val() * $kdv.val());
});
</script>