Some additional notes: Custer State Park is a state operation and as such, requires a different sticker than a Nat Parks Pass. I think its $10, but its worth it. Needles Highway is a great drive, but not for RVs or TTs. Some of the tunnels thru the mountains will be your permanent home. Drive the TV or toad. The wildlife loop is a must, but early morning or late afternoon is best for viewing the wildlife as the heat of the day drives them into the trees - except the wild burros. They're descendants of the old prospector's burros and they run an interesting scam. One walks out in the road to block traffic and the others go from car to car, begging for handouts. Take carrots, and watch your fingers.
Mt. Rushmore is a National Park, so your pass gets you in for free, but the parking facility was financed by the people that live in the Black Hills, and until its paid off, there is a fee for parking. Its still worth it. Go during the day and return at night for the lighting ceremony. The parking pass is still good.
Unlike Rushmore, Crazy Horse is a private operation, and was requested by the leaders of the Lakota Sioux to honor one of their great chiefs. As such, there is an entrance fee, $15 I think, but part of it goes to finance college scholarships for the Native Americans that live in the area. You can pay extra for a ride to the top if you choose. I did, and the view is spectacular. Crazy Horse's arm is as wide as a four lane highway.
