javascript sum function + tax

i have a one problem. please help me.

i have a javascript code but doesnt work now.

that code:

<input type="checkbox" id="konut1" name="tasinmazsecim" class="tasinmazsecim" value="489" /><span>1-100m2</span> 
    <span id="payment-total" style="text-decoration:underline;">0</span>
    <input id="input" type="text" value="0"/>


    window.onload=function(){
var inputs = document.getElementsByClassName('tasinmazsecim'),
    total  = document.getElementById('payment-total');

 for (var i=0; i < inputs.length; i++) {
    inputs[i].onchange = function() {
        var add = this.value * (this.checked ? 1 : -1);
        total.innerHTML = parseFloat(total.innerHTML) + add
        var new_total = parseFloat(document.getElementById('input').value);
      console.log(new_total);
        document.getElementById('input').value=new_total + add
    }
  }
}

checkbox value + other checkbox value this worked.

but i need checkbox value + 0.07 tax + 50

please help me.