But once the technology is developed and available the cost will probably be pretty low. A couple of sensors and processing. I feel like maintenance cost will likely be smaller since a self driving car in theory at least can detect any issue much quicker then a human.
This is all speculation of course but human drivers are very expensive. Quick googling says about 20k to 45k per year for a truck driver. Even if initial investment in self driving technology costs 50k per unit more it's still incredibly advantageous to do so.