I knew someone who drove a taxi, before Uber was available, and he said the biggest cost is car repairs as they ride them hard each day. Then there is variables like gas and insurance which fluctuate. These costs apply to all drivers, not just Uber. He said he made around $45k/yr if I remember correctly. But he said the freedom of having no boss and driving around meeting people made it worth it.
This will probably depend on where the driver lives and works. There is also the question of whether they are a 'broker' or not with another taxi, meaning they own their own car and also do Uber and/or Lyft on the side while working with another taxi service. Which is common.
In Toronto where our taxis are far less regulated than other cities like New York, allowing for more supply but higher per ride fares to compensate, all of the drivers I've spoken to have said they make more money per ride with Uber than they do with normal taxi services and always give preference to Uber calls over the dispatcher. Also they have been busier allowing them to do more rides per night worked.
Uber likely increases the amount of people taking rides in any city as it makes the UX easier to call a cab reliably, direct it, and pay for it.