Because the main cost of an uber is the driver's wage, so I think it could be much less than half. Then if you share the car with multiple riders per trip, that could lower the cost much further (at the expense of slightly longer trips.)
I expect that cities will heavily subsidize it, to bring it down to about the level of public transport (but far more convenient). I'll admit it's just guessing at a very nebulous future, though.
Why do you think it's going to be dirt cheap? I expect it's going to be half of an Uber ride, but not much less.