Rakuten Real Price.user.js

// ==UserScript==
// @name         Rakuten Real Price
// @namespace    https://gist.github.com/closer
// @version      0.1
// @description  楽天市場の商品価格ナビでポイント還元を加味した価格に補正します。
// @author       closer
// @match        https://product.rakuten.co.jp/product/-/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    // Your code here...
    document.querySelectorAll('.specColumnUsed').forEach((a)=>{
        const priceElm = a.querySelectorAll('.itemPrice3')[0];
        const pointElm = a.querySelectorAll('.hasPtPopup')[0];
        const price = parseInt(priceElm.innerText.replace(',',''));
        const point = parseInt(pointElm.innerText.replace(',',''));
        const discount = price - point;

        const discountElm = document.createElement('span');
        discountElm.className = 'UsedTxet01 itemPrice3';
        discountElm.appendChild(document.createTextNode(discount.toLocaleString()));

        const halfyen = document.createElement('span');
        halfyen.className = 'halfyen';
        halfyen.innerText = '円';
        discountElm.appendChild(halfyen);

        priceElm.before(discountElm);
        priceElm.before(document.createElement('br'));
        priceElm.className = null;
        priceElm.innerText = `(元値)${priceElm.innerText}`;
    });
})();