*Correction*
Driver–$60, Passenger–$35, and Under 12–$15.
Also on the registration page, you can order Long Sleeve Tesla Rally Shirts. Late registrants won't be able to order the shirt.
Quoting the registration page, "Each registered driver will receive $10 in Custer Cash, a 2020 Tesla Pin, and a Crazy Horse Memorial dash ID that will get you free entrance all day on Saturday." The advantage to registering early is to insure that you will receive the 2020 Tesla Pin. The folks at the Custer Area Chamber of Commerce can print more Custer Cash and Crazy Horse Memorial dash IDs, but once the pins run out, you are out of luck. Registering early also helps give the businesses time to order food or other supplies.