[рони]

Taichi,

function FullPay()
{
     var aa = document.querySelectorAll('form.intake input[name="aa[]"]'),
          bb = document.querySelectorAll('form.intake input[name="bb[]"]'),
      cc = document.querySelectorAll('form.intake input[name="cc[]"]'),
      full  = document.querySelector('.FullPay'),
      sum = 0;
     for (var i = 0; i < aa.length; i++) {
     cc[i].value = (+aa[i].value||0) * (+bb[i].value||0);
       sum += +cc[i].value;
  };

   full.innerHTML = sum
}