I actually have a very good idea of the operating cost of the E190 vs the 737 its not a huge difference in the grand scheme of things. Fuel burn yes the E190 is much better but other areas cost more, the 737 is like the Ford Falcon of aviation, its cheap, cheap parts, mass available spares, economy of scale etc etc. The EJets are a bit like a Euro car made in Brasil, everythings a little more expensive but it burns less fuel. A bit like the ATR 72 vs the Q400, the Q400 is cheaper to maintain and can do more sectors per day, as well as longer TBOs, the ATR uses much less fuel, but just about everything else costs more due to lower component times etc etc.
RJs are lower cost to operate thin longer routes or in particular in Australia the FiFO market. WRT the use of them as regional airliners here, they only work if near full constantly, otherwise the Q400 is way better economics wise. The NJS 717s are probably experiencing more maintenance issues due to overextending times when things should have been replaced, that's usually a sign they are trying to get blood from a stone economics speaking.