var list = $$("span.a-size-base.a-color-price");
var total=0;
for(i=0; i<list.length;i++){
  var n = parseFloat(list[i].innerText.replace("£","").replace(",",""));  
  if(n === Number(n)) total += parseFloat(n);
}
alert(list.length+" items for a total of: £"+total.toFixed(2));