I have a yeti 45qt that was given to me for Christmas one year, and I love it. I'm not sure I would buy one for myself at full price, mainly because I am cheap like that...
However, to answer the original question, the price comes from the construction and method of manufacturing. Yeti's are roto molded, as compared to other plastic manufacturing process (injection, blow molding, or thermoforming). Plain and simple, roto molding is a long, dirty, and expensive process, but it allows for the product to have a much thicker side wall, and therefore is much stronger than coolers made from traditional manufacturing methods. This is what gives the yeti and other similar coolers the ability to be run over and withstand the weight of a jeep.