Just A Bit

<div id="container">
  <input type="checkbox" name="price-input" value="10" onClick="calculate(this);"  />10<br />
  <input type="checkbox" name="price-input" value="20" onClick="calculate(this);" />20 <br />
  <input type="checkbox" name="price-input" value="30" onClick="calculate(this);" />30 <br />
  <input type="checkbox" name="price-input" value="40" onClick="calculate(this);" />40 <br />
</div>
Sum : <span id="sum">0</span>

var total = 0;

function calculate(item) {
  if(item.checked){
    total+= parseInt(item.value);
  } else {
    total-= parseInt(item.value);
  }
  document.getElementById('sum').innerHTML = total;
}