If you just sit on the road and look at it, you're missing the historical stuff inside and the video which talks about the creation of the sculpture. Both of those really made the place for us. The more familiar you are with Lakota history and culture, the more you'll get out of it. It was especially neat looking at the photos of the survivors of the Little Bighorn. Yes, it should be a little further along, but I'm not a rock carver and when you see (from the video, the one that you really need to watch) how much as already been taken off, it starts to make sense.
Entrance was $27 for a family of 5. That's just over $5/each. Cheap. We spent 2 hours inside and left only because we weren't going to make Rushmore and back to the park before dark. I bought nearly $100 worth of books from the bookstore and was glad to get them as some of them are really hard to find.