changePrice

  function getParameterByName(name, url) {
    if (!url) url = window.location.href;
    name = name.replace(/[\[\]]/g, "\\$&");
    var regex = new RegExp("[?&]" + name + "(=([^&#]*)|&|#|$)"),
        results = regex.exec(url);
    if (!results) return null;
    if (!results[2]) return '';
    return decodeURIComponent(results[2].replace(/\+/g, " "));
}

function changePagePrice() {
  var spanPrice = document.getElementsByClassName('woocommerce-Price-amount') 
  spanPrice = spanPrice[spanPrice.length -1]

  var price = parseFloat(spanPrice.textContent.replace(/\$/i, ''))
  var days = parseInt(getParameterByName('days'))
  var newPrice = '$' + (price * days)
  spanPrice.textContent = newPrice
}

changePagePrice();