The additional hardware and software will be expensive as hell to develop, but once it's invented, the marginal cost should be fairly cheap compared to the price of a car.
processing power = cheap
software (marginal cost) = just gotta pay for updates and fixes
sensor package = expensive now because nobody is making 6 million of them a month. Once they do, it'll be a lot more affordable.
That doesn't mean it'll pay off for Google, uber, Telsa, Ford, etc., but if they build it, it'll move to all cars rather quickly.