"In 2010, there were 1.1 fatal crashes per 100 million truck miles"
https://www.truckdrivingjobs.com/faq/truck-driving-accidents... Truckers make ~27c/mile.
So, at similar rate to human drivers that's ~24* million dollars per fatal crash in saved wages. Plus presumably higher truck utilization and lower management costs etc. Don't forget the driver is often not at fault and automated trucks should have plenty of footage of accidents to demonstrate who was at fault. So it's a fraction of both fatal and non fatal accidents, with non fatal accidents generally having lower associated costs. Also, most importantly it's only the truck involved in many accidents associated with bad weather and more generally the driver's medical bills etc do not need to be covered if there is no driver.
Further, technology is unlikely to degrade over time so they just need a profitable starting point. Also, if the self drive button cost ~25c / mile there are plenty of times I would hit it.
*Granted, there will be labor costs linked with automated trucks, but a 95% savings seems likely.