For those interested, the Aviation Analytics website has a free to use Route Economics Calculator that features FlyBe as well as other leading European airlines. You can quickly set up their new routes from LCY, SEN, BHX , choose an aircraft type (DH4 or E75) and view the estimated break even load factor against a given fare and vica versa.
I have just run the costs for Flybe on LCY-DUB on a Dash 8.
Looks like sector costs of around £4000 (assuming airport charges of £5 per pax or £10 per departing pax)
That's a break-even fare of £78 o/w including ancillary revenues at 65% break-even which would need 51 passengers per sector on average.
Not sure how this compares to CityJet (the calculator only has Flybe of the two) but my guess would be a significantly higher sector cost for the ARJ meaning many more passengers to break-even at the same fare (even if the total average seat costs are slightly lower).