At which point, your costs = costs of owning a car for each commuting user in your area + overhead + profit. So unless you are offering subsidized prices, it will be cheaper for everyone to just own their own car for commuting.
You're also forgetting that most existing cars on the road have way more seating capacity than needed, since they need to account for the maximum number of people who will use that car at once. A family of four needs a car with four seats, even if most of the time that car will only be carrying one person to work and back throughout the week. Self driving cars will be much more efficient in that respect.