[sanmihan]

<form onsubmit="some(this)">
Сумма заказа: <input name="summ" type="text" value="200"><br>
<input type="submit" value="Оплатить">
</form>
<script>
function some(form){
  var s = +form.summ.value;
  s = s + s*0.025;
  alert(s);
}
</script>