Casm of 23 seems way high to me. IIRC CoffeeBean quotes casm for Jazz of something like 28 cents with Jazz operating a fleet of far less efficent aircraft than the Q400. My guess is that Porter will be operating with far less overhead than Jazz and possibly less than WJ. Factor in presumably far lower landing fees at the Island than YYZ and Porter will almost certainly be the lowest cost carrier by a country mile.
If Bombardier's web site is to be believed, break-even load factor for a 300 NM trip (roughly YTO - YUL) charging an average $100 (US?) per seat is 35%. See
http://www.q400.com/q400/en/turbo.jsp
If these numbers are to be believed, don't count them out too soon.