[Vlasenko Fedor]

<form name="9mmammo">
  <input type="text" class="calc" value="1" data-operator="*" data-val="5">
  <div>Сумма:
    <span class="result">5</span>руб.</div>
</form>
<form name="556ammo">
  <input type="text" class="calc" data-operator="*" data-val="10" value="1">
  <div>Сумма:
    <span class="result">10</span>руб.</div>
</form>
<form>
  <input type="text" class="calc" data-operator="+" data-val="3" value="1">
  <div>Сумма:
    <span class="result">10</span>руб.</div>
</form>
<script type="text/javascript">
  var inputs = document.querySelectorAll("input.calc");
  var len = inputs.length;
  while (len--) inputs[len].onkeyup = showResult;
  function showResult() {
    var val = this.value = this.value.replace(/\D/g, '');
    if(!val) return;
    var operator = this.getAttribute("data-operator");
    var opval = this.getAttribute("data-val");
    var par = this.parentNode || this.parentElement;
    var rez = par.querySelector("span.result");
    rez.innerHTML = Function('return ' + val + operator + opval)();
  };
</script>

миникалькулятор