Price difference is not a reflection of the amount of nutrition in the formula. All formulas must meet strict FDA guidelines for adequacy of nutrition content. From that perspective, all formulas will meet your baby's nutritional needs. It could be that the source of protein is causing the price differential. As long as the nutritional requirements are met, the companies have the freedom to change the ingredients they use in their formulas.
Ingredients can be a big factor in formula cost.
According to the tables in the text published by the American Academy of Pediatrics, the major protein source in Enfamil and Similac is casein. Carnation uses mainly whey protein. The source of protein will not affect the nutritional content of the formula. Watch this report about showing affection to boys. Find out here.