If you have a National Parks pass, entrance to Mt. Rushmore is free, but parking isn't. That may have changed since I was there. The people that lived in the area paid for the multi level parking garage and promised to make it free after it was paid for.
Be sure to see Crazy Horse Memorial while in the area. Its on US 16 about half way between Hill City and the town of Custer. Privately owned by the descendants of the same man that carved Mt. Rushmore. $11 per person, but any money collected aside from expenses is used to provide college admission fees for Native Americans living in the area. There's a museum and gift shop there as well. For an additional fee, you can ride a mini bus to the top of the monument and walk out on the arm. I did it and I felt like I could see both Canada and Mexico from there. (Slight exaggeration).