That is the cost of working around 50 years of radioactivity buildup on all of those pumps, pipes, and valves in eight reactor plants. It must all be safely removed and put in secure storage before the other parts of the ship can be broken down for scrap.
Don’t forget about inflation: $451.3M in 1955, is now worth $3,976,138,235.07.
The cost to decommission the ship, is but a small fraction of what it cost to build.