Small user script to be used with something like TamperMonkey that will add a tooltip to the "x receives y per week from z patrons" that provides the amount per month. Currently only works with the english locale of liberapay.

liberapay_per_month.js

// ==UserScript==
// @name         Liberapay donations per month
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Hovering over the "user receives ... per week from ... patrons" will now show the received amount per month. Currently only works for the english version since it matches the header line via the contained text - couldn't find better attributes for that.
// @author       Gina Häußge <osd@foosel.net>
// @match        https://liberapay.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    var element = $(".profile-header-bar div.flex-col:contains('per week from')");
    var amount = element.find("b")[0];
    var perWeekText = amount.innerText;
    var perWeek = Number(perWeekText.replace(/[^0-9\.-]+/g, ""));
    var perMonth = perWeek * 52 / 12;

    var perMonthText = String(Math.round(perMonth * 100) / 100);
    if (perWeekText[0] === "€") {
        perMonthText = "€" + perMonthText;
    } else if (perWeekText[0] === "$") {
        perMonthText = "$" + perMonthText;
    }

    element[0].title = perMonthText + " per month";
})();