Calculates the monthly payment, with taxes and HOA if applicable, for a listing on cleanoffer.com. This script is San Francisco Ready(tm) in that it will show you the list price, as well as the actual price (estimated at 20% over list).

cleanoffer.pmt.user.js

// ==UserScript==
// @name show mortgage cost on cleanoffer
// @author mdaniel
// @version 1.7.0
// @namespace http://MatthewDaniel.com/greasemonkey/cleanoffer.com
// @include http://re.cleanoffer.com/b/zephyr/mybriefcase/viewnewmatch.htm*
// @match   http://re.cleanoffer.com/b/zephyr/mybriefcase/viewnewmatch.htm*
// @include http://re.cleanoffer.com/b/zephyr/listing.htm*
// @match   http://re.cleanoffer.com/b/zephyr/listing.htm*
// ==/UserScript==

/**
 * The "percent" APR for your 30 year fixed mortgage.
 * @type {Number}
 */
var MORTGAGE_APR = 3.875;
/**
 * Just what it says.
 * @type {Number}
 */
var DOWN_PAYMENT = 300000;
/**
 * The property tax "percentage".
 * @type {Number}
 */
var TAX_RATE = 1.188;

/**
 * Poached from http://stackoverflow.com/a/22385930.
 * @param {Number} ir   - interest rate per month
 * @param {Number} np   - number of periods (months)
 * @param {Number} pv   - present value
 * @param {Number} fv   - future value
 * @param {Number} type - when the payments are due:
 *        0: end of the period, e.g. end of month (default)
 *        1: beginning of period
 */
function PMT(ir, np, pv, fv, type) {
    var pmt, pvif;

    fv || (fv = 0);
    type || (type = 0);

    if (ir === 0) {
        return -(pv + fv)/np;
    }

    pvif = Math.pow(1 + ir, np);
    pmt = - ir * pv * (pvif + fv) / (pvif - 1);

    if (type === 1) {
        pmt /= (1 + ir);
    }
    return pmt;
}

// wrap in a function for easier try-catch
var injectCosts = function() {
    var priceH3NL = [].map.call(
        document.querySelectorAll('h3'),
        function(it) {
            var ma = /Price: *[$]([0-9,]+)/.exec(it.innerHTML);
            if (!ma) return null;
            return [it, new Number(ma[1].replace(',', ''))];
        }).filter(function (it) { return null != it; });
    if (0 === priceH3NL.length) {
        return;
    }
    var h3 = priceH3NL[0][0];
    var cost = priceH3NL[0][1];
    var mortgages = [cost, cost * 1.20].map(
        function(it) {
            return -1 * PMT(
                (MORTGAGE_APR / 100) / 12,
                30 * 12,
                it - DOWN_PAYMENT
            );
        }
    );
    var taxes = (cost * (TAX_RATE / 100)) / 12;
    var duesList = [].map.call(
        document.querySelectorAll('#listing-details .hd')
        , function (it) {
            if (null == /Dues/.exec(it.innerHTML)) {
                return null;
            }
            var numberTD = it.parentNode.querySelector('td ~ td');
            if (null == numberTD) {
                return null;
            }
            return new Number(numberTD.innerHTML);
        }).filter(function(it){ return null != it; });
    var dues = (0 === duesList.length) ? 0 : duesList[0];

    var mortgage1str =
        '     $'+(mortgages[0] + taxes + dues).toFixed();
    var mortgage2str =
        ' +20 $'+(mortgages[1] + taxes + dues).toFixed();
    var hoastr = (0 === dues) ? '' : ' HOA $'+dues;

    var s = document.createElement('span');
    s.style.color = 'green';
    s.style.paddingLeft = '20px';
    s.appendChild(
        document.createTextNode(
            mortgage1str + mortgage2str + hoastr
        )
    );
    h3.appendChild(s);
};

try {
    injectCosts();
} catch (ex) {
    console.error('Kaboom', ex);
}