As George Tower has mentioned, at SA Express we operate the CRJ, DH8-300 and Q400.
For calculation purposes, you can use the following numbers for the 3 aircraft:
CRJ: TAS 420KTS, Burn (1st hour) 1500kg, 50 seats
-300: TAS 260KTS, Burn (1st hour) 700 KG, 50 seats
Q400: TAS 360KTS, Burn (1st hour) 1200 KG, 74 seats
Obviously there are other factors:
Runway requirements (we can only fly 300 into RCB)
Services (Copco, GPU, hot catering)
Purchase cost
Maintenance
Competition on route/Passenger acceptance
The CRJ is the most expensive to maintain (due to complex systems, ie. Oxygen and powerful APU for bleed starts), followed by the Q400 (full EFIS and very powerful engines) and then the 300.
Crew utilisation is another factor. If you operate a slow turboprop on long sectors, you might only be able to use your crew for 2 sectors, whereas a jet crew would be able to do the same 2 sectors and have duty left for another 2.
Frequency required to destination. Obviously a jet can do more trips to the same destination (lets say over 300nm) than a turboprop.
BTW, although the Q400 is 60KTS slower than the CRJ in the cruise, it can maintain the same speeds in the TMA and the time difference on sectors under 350nm is negligeble. Because of the extra 24 seats, the cost per seat mile is much better.
Hope my incoherent post helps.