[рони]

Lemme,
быстрее будет без циклов

<form name="form" action="http://">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<input class="chek" type="checkbox">
<div  class="rezultat"></div>
</form>

<script>
window.addEventListener("DOMContentLoaded", function() {
    var a = document.querySelector("form"),
        b = document.querySelector(".rezultat");
    a.addEventListener("change", function() {
        var c = a.querySelectorAll(":checked").length
        b.innerHTML = "Total: " + (c > 5 ? (c - 5)*5 + 50 : c * 10)
    })
});
</script>