Harley-dave is right about Sturgis. While the prices on Custer SP will not go up, there will be no sites available either. Other sites in the area for several miles around will go up in price considerably. Unless you want to specifically be in or around the Black Hills during the Rally, it might be a good idea to route yourself to another part of the country after then (or perhaps early, but things get busy as early as a couple of weeks prior to the Rally) The Hills are much prettier in the spring before the prairies dry out later in the summer - for whatever it might be worth to you.