function checkAll(checkname, exby) {
  for (i = 0; i < checkname.length; i++)
  checkname[i].checked = exby.checked? true:false
  onclick=sumup();
}

function sumup() {
var sum = 0;
for (var i = 0; i < document.f.field.length; i++) if (document.f.field[i].checked) {
sum += parseFloat(document.f.field[i].value); }
document.getElementById("sum").innerHTML = (isNaN(sum) ? 'Geen getal.' : sum);
};