You still have to pay someone to fly the plane. Spread that cost among four passengers, and you ain't gonna get a competive fare. Look at the cost just to ride in a taxi...
Unfortunately the 500 needs a new engine, as the Williams unit proved underpowered. It hasn't made a flight since its first one earlier in the year. I still think the concept is solid, and if Vern Rayburn can't get it in the air, I don't know of anyone else who could.