I don't get it - is the unit cost per driving system cheaper than the cost of one human driver? Surely in a huge operation like that, the costs of operating a giant multimillion dollar vehicle could also include the salary of a driver?
Autonomous trucks will get to the stage where they simply don't stop. Human operated trucks need breaks, and make mistakes. So it doesn't matter if they cost more - they also produce more.
Finally, autonomous trucks don't cause OH&S incidents nearly as easily. This is a huge issue, especially for a company like Rio Tinto. In Australia at least, OH&S issues can shut mines down. This alone would probably be worth any price premium in the million dollar order of magnitude.