Is Costco really worth its premium price tag? These two business trends can justify the retailer's lofty valuation.Warehouse retailer Costco Wholesale (COST 0.25%) may sell goods at affordable prices, but the stock is pretty expensive. Costco investors have pocketed a total return of 2,320% over the last 15 years, leaving the S&P 500 index far behind at a 663% gain. The stock traded at a luxurious 55.8 times trailing earnings on June 26, or 59.6 times free cash flow. So you wouldn't be the first investor to ...