Copy, paste and run in dev console.

amazon-wishlist-discount-checker.js

(function() {

const THRESHOLD = 400;

function isEndOfList() {
  return document.querySelector("#endOfListMarker") !== null;
}

function scroll(top) {
  document.scrollingElement.scrollTop = top;
}

function scrollToLastItem() {
  const top = document.querySelector("#g-items").getBoundingClientRect().bottom
    + document.scrollingElement.scrollTop;
  scroll(top);
}

function isWishListPage() {
  return document.querySelector("#wl-item-view") !== null;
  // or we can also check the URL.
}

function loadAll(cb) {
  if (!isEndOfList()) {
    scrollToLastItem();
    setTimeout(() => loadAll(cb), 1000);
    return;
  }
  cb();
}

function getItems() {
  const elItems = document.querySelectorAll("#g-items [id^=item_]");
  return Array.from(elItems).map(el => {
    const elName = el.querySelector("[id^=itemName_]");
    const elByline = el.querySelector("[id^=item-byline-]");
    const elPrice = el.querySelector("[id^=itemPrice_] .a-price-whole");
    return {
      name: elName.textContent,
      byline: elByline.textContent,
      price: elPrice.textContent,
    }
  });
}

function main() {
  if (!isWishListPage()) {
    console.error("This page does not contain the wish list.");
  }

  loadAll(() => {
    getItems().forEach(item => {
      if (item.price < THRESHOLD) {
        console.log(item);
      }
    })
  });
}

main();

})();