The lunar rover, carried by Apollo 15 to moon, cost 60 million USD, in 1971. From a layman's view, it was basically a set of 4 motorized wheels, perhaps very lightweight and with longer battery life and with sophisticated communication equipment. That is far more than the costliest four wheeler even in 2021. What was so special about it (other than the honor of traveling on the Moon), that it was so costly?
Even if the number you quote is correct, I believe that is the cost of the entire rover program.
That not only includes the cost of the 4 rovers built, but the cost to develop and test them, plus all the extra versions that were needed. One doesn't just make a rover and say here you are. It has to be integrated into the rest of the systems. They would have had versions that were used to train the astronauts. One would have been used to test the deployment system. Another would have been used to make sure everything fit in the launch system. One would have been used to test all the electrical systems. They may have used another to test in the vacuum chamber, or the shaker table to make sure it survived the launch and the flight to the moon.
Most of the cost would not have gone into parts and assembly. It would have been all the development and testing costs.