The E175/Q400 cost debate is an interesting one. It's also not new. See
this article from back in 2011 which summarises their thinking at the time.
Essentially flybe recognised that the E175 was naturally going to be more expensive to operate when they got them (as the comments above about fuel burn confirm), but they managed to reduce some costs (e.g. engine maintenance) and they also got a very good financing deal (via the Brazilian export credit agency) which meant they could buy the aircraft (and pay low interest rates for the loan) rather than leasing them.