ATR 72 vs. Q400
The two aircraft are built for very different missions, and operating cost will depend greatly on the stage-length used.
The ATR 72 is the lightest airframe in existence on a per-seat basis, which means that it will have low MTOW driven costs (airport & navigation fees) coupled with a low fuel consumption. This will show it's full advantage on short routes 700kms or less) in a high airport/atc cost environment (eg. Europe). However as it has a low cruise speed, on longer routes its advantage will disappear due to the increased crew and maintenance costs which tick on a flight hour basis.
The Q400 on the other hand has a higher mtow, but almost jet-like speed coupled with turboprop fuel econimics. On routes between 1000 and 1500 kilometres it has unbeatable seat-economics (route costs of a Q400 match that of a CRJ-200, but it has 20+ seats).
The most important message is that you cannot compare two aircraft on an average cost basis. You need to look at specific routes to get a meaningful comparison. There is an optimum for every aircraft, and even this depends on the cost regimes of the particular region.