AirInsight published a very detailed report about the CSeries this week. The main question to be answered was, if, after Airbus announced the A320NEO, there would be still a business case for the Bombardier CSeries. Not to my surprise (because of own studies), the answer was: Yes!
The only really big threat for the CSeries, according to the report, would be pricing-power from Airbus and Boeing, which could choose to give away A319NEO and 737-700W (or maybe also a 737-700NEO???) with losses just to protect their market.
There is an additional short piece at AirInsight detailing that threat here.
So how deep has a discount to be to be attractive?
Let's start with the price of the aircraft: let's start with the 41% that Airbus has to give according to the AirInsight article to match a 25% discount by Bombardier.
The AirInsight report comes to the conclusion that for a 500nm mission the CS300 will be $716 lower in costs and $444 lower than a A319NEO. I do not fully agree with the number of seats the report puts down for the various aircraft, so I will not discuss the differences in seat-costs here. Just assume a given flight with a given number of passengers (at or below the maximum of the smallest of the three aircraft), no matter what aircraft you fly.
For an airline, to get a better deal over the full life of the aircraft one should not only take into account the purchase price of the aircraft but the overall costs including operation and maintenance of the aircraft.
Assume there a seven flights per day on 320 days per year (allowing for maintenance) for 10 years - that makes 22,400 flights.
So the difference in operating costs between a CS300 and a 737-700W for these 10 years would be about $16 million, about $9.5 million against the A319NEO.
A big portion of the difference in operating costs is fuel burn - and if the price of fuel goes up ($2.25/gallon was assumed in the study), the operating cost difference goes up as well.
So there would be another big discount needed to make up for those difference in operating costs. If Airbus and Boeing are willing to make these discounts remains to be seen, but I doubt it - although Airbus once made such discounts for the A340-600 to sell it against the B777-300ER.