PriceConverter.js

prices = document.getElementsByClassName('preis');
for(i in prices) {
  var price = prices[i];
  price.innerText = 'ab ' + Math.round(
    parseInt(
      price.innerText.substring(3)
      )*0.8) + ',-€';
}
  
prices = document.getElementsByClassName('preis');
var reg = /(\d+)/;
for(i in prices) {
  var price = prices[i];
  if(reg.test(price.innerText))
    price.innerText = price.innerText.replace(reg,
      Math.round(
        parseInt(
          reg.exec(
            price.innerText)[0])*0.8));
}