Sometimes, even great, growing companies cost too much to buy. Costco Wholesale Corporation (COST -1.53%) investors had a good week. From last Friday's close below $744 through Friday, May 10, shares of the warehouse retailer gained roughly$40 in value after reporting strong e-commerce sales in its update for April. And $40 could be just the beginning. Loop Capital analyst Laura Champine predicts that, within a year, Costco shares will fetch$840 apiece. Is Costco stock a buy? Costco reported Wednesday tha ...