For the most part they aren't cheap because of the liability and that it is a mandated program. There is no incentive for the companies providing the service to reduce charges because someone is being told they have to pay whatever price is quoted -no matter the price-. There is an argument to be made that it shouldn't be cheap - there is a lesson being taught after all. That is beside the point, though.
The actual hardware is pretty inexpensive and pretty hardy. It is in an inhospitable environment without environmental controls and in a vibration-prone mounting. The real cost is the consumables, which are paid by the users.
Directly reading BAC through the back of a watch or with a pinprick type reader ala diabetes testers might be messy, and certainly would need to run through numerous trials.
It might be very expensive to get to market, come to think of it.
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