I am in no position to be able to know with any certainty if it is cheaper to make a conventional trunk opening like on the Model 3 compared to making a hatchback type opening link on the Model S. I can only guess that there are some cost savings, since to me a hatchback has greater utility. It is the only feature of the Model 3 that I am disappointed about (based on the limited amount of Model 3 information available so far).
My only major concern about the Model 3 compared to the S is what the largest battery size option will be on the 3. Since the Model 3 wheelbase is 3" less than the Model S, the room available for the battery pack is reduced.
I want to tow a 17 ft 2,000 lb camper trailer with the Model 3. To achieve any sort of useful towing range, which to me is at least 150 miles at 55mph on a level dry road with no headwind, I want as much battery capacity as it is possible to fit into a Model 3.