Made the original script into a user script to be used with GreaseMonkey or Tamper monkey. Remove the alert. Instead this outputs the total at the top of the list along side the list controls.

gistfile1.js

// ==UserScript==
// @name       Amazon Total Cost of Wishlist
// @namespace  http://www.amazon.com/registry/wishlist/
// @version    0.1
// @description  Total cost of amazon wishlist
// @require http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js
// @match      http://www.amazon.com/registry/wishlist/*
// @match      http://www.amazon.com/gp/wishlist/*
// @copyright  2012+, Jack
// ==/UserScript==

var list = $("#item-page-wrapper .list-items table tbody .lineItemMainInfo .lineItemPart strong");
var total=0;
for(i=0; i<list.length;i++){
  total += parseFloat(list[i].innerText.replace("$",""));
}
$("span#printThisList").after(list.length+" items for a total of: $"+total.toFixed(2));