Well they do have to truck in everything to a National Park, and that costs someone to use a lot of fuel, so they do have to charge more. And most National Parks charge the store rent - usually an outrageous amount too!
Have you heard of Flagg Ranch? it is between Jackson Hole and Yellowstone. It is a nice full hookup park, but a bit crowded for my liking, as I usually dry camp. 2 miles to the west are several dry camping areas. Pick one, and enjoy the view of the Snake River (or creek, as it is only a few feet wide at this point near it's start).
Have fun camping!
Fred