Why would they make it cheaper when they can make even more profit by not having to pay a voice actor?
Usually on balance this falls somewhere in between -- more value for less money for the consumer, and more profit on each marginal unit of production for the producer, which is how technology progresses across most consumer goods.
This opens up for non-signed authors to release audio books.
Unless you're talking about illegal niche, in which fair but I highly doubt stores are going to accept those. All generation helps with is content that will be free.