[рони]

sergylt,

<form name="form" action="http://">

 <fieldset class="checkbox_buy1">
 <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">
 </fieldset>

 <fieldset class="checkbox_buy1">
 <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">
 </fieldset>

 <div class="rezultat"></div>

</form>

<script>
window.addEventListener("DOMContentLoaded", function() {
    var a = document.querySelector("form"),
        b = document.querySelector(".rezultat");
    a.addEventListener("change", function() {
        b.innerHTML = "Total: " + [].reduce.call(a.querySelectorAll("fieldset"), function(a,b) {
            var c = b.querySelectorAll(":checked").length;
            return a + (c > 5 ? (c - 5)*5 + 50 : c * 10)
        }, 0)
    })
});
</script>