Pladzuma,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
window.addEventListener('DOMContentLoaded', function() {
var form = document.querySelector('form');
form.addEventListener('input', function() {
var price = +form.price.value || 0;
var amount = +form.amount.value || 0;
form.sum.value = price * amount
});
});
</script>
</head>
<body>
<form>
<div>
<span>Цена товара</span>
<input name="price" type="text"></input>
</div>
<div>
<span>Количество</span>
<input name="amount" type="text"></input>
</div>
<div>
<span>Сумма транзакции</span>
<input name="sum" type="text"></input>
</div>
</form>
</body>
</html>