From an opinion piece in 2012 — http://www.nytimes.com/2012/05/26/opinion/they-are-taxis-not... — drivers made around $113/12 hour shift after paying the rental fee (also $113). There was a related news article around the same time (this was in discussion of a fare hike) but I can't find it.

Typically drivers who rent a taxi rent by the shift and must pay their rental fee upfront. There are ghost fleets of taxis queued up at garages in Brooklyn and Queens since so many drivers have dumped medallion cabs for driving for Uber/Lyft/etc.