Before or after you visit Mount Rushmore National Memorial you have to visit Crazy Horse Memorial, about 25km to the west in South Dakota.
Crazy Horse Memorial was ordered by Oglala Lakota Chief Henry Standing Bear. He asked Korczak Ziolkowski to create a monument honoring all North American Indians. Ziolkowski was chosen for this job because of his work on Mount Rushmore.
The Crazy Horse Memorial is larger than Mount Rushmore, but is far from finished. Only the face was finished in 1998, 50 years after the start of construction. The rest of the monument is still a work in progress. The monument is not part of the National Parks System and is financed by admission fees and contributions.
Actual information and prices can be found on the website: https://crazyhorsememorial.org