Realty Income looks really cheap these days.The S&P 500 currently trades at nearly 23 times forward earnings. That is a historically high level, as the market has traded at an average of around 15 times forward earnings over the past couple of decades. While the market is currently pricey, that's not the case with Realty Income (O -0.34%). The real estate investment trust (REIT) trades at a much lower level compared to the market and its peer group. That makes it a no-brainer buy in today's market. A dirt c ...