<h1><script>
  function test()
  {
    var x = document.getElementById('x');
    var r1 = document.getElementById('r1');
    var r2 = document.getElementById('r2');
    var r3 = document.getElementById('r3');
    var v = document.getElementById('v');
    //
    var suma=0;
    if (r1.checked) suma=500;
    if (r2.checked) suma=700;
    if (r3.checked) suma=900;
    if (x.checked) suma+=(suma*0.23);
    v.value = suma;
  }
</script>
<form>
  <input type="radio" name="r" id="r1"> i3 = 500zł<br>
  <input type="radio" name="r" id="r2"> i5 = 700zł<br>
  <input type="radio" name="r" id="r3"> i7 = 900zł<br>
  <input type="checkbox" id="x"> VAT 23%<br>
  Razem: <input type="text" id="v"><br>
  <input type="button" value="Oblicz" onclick="test()">
</form>