Costco is a strong dividend growth stock, but investors have pushed the price near all-time highs. Realty Income is in a different position.On the surface, retailer Costco Wholesale (COST -0.77%) and real estate investment trust (REIT) Realty Income (O 0.57%) have little in common.One of the biggest differences between these reliable dividend stocks today is found on the valuation front, with Costco looking historically expensive and Realty Income relatively cheap. A lot of investors, particularly those in ...