$40k to outfit a truck for self driving would be really cheap. The safety requirements for a computer really handle a vehicle like that will probably require a much greater investment. Drivers need to be able to do things like hear breaks screeching, feel the thump when they lose a retread from their tire, feel a flat tire pulling them, etc. To do away with the driver sensors need to be integrated into almost every point of failure. This would suggest that a retrofit is far less likely then simply building new trucks, which I'd guess to be more in the range of $400k.
I'm guessing we'll see self driving trucks phase out human drivers gradually by replacing trucks which are due to be replaced anyway. Even if self driving trucks save you the cost of a salary it's still profitable to drive your existing trucks into the ground with human drivers.