If a trip costs $10,000 to operate and is 1000nm and the aircraft has 200 seats. The CASM is 1000(nm)X200(seats)=200,000(ASM).
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$10,000 / 200,000ASM = $0.05 CASM
The post above about CASM ruling all is FALSE. There are several other factors that factor in, namely RASM, or Revenue per ASM. A rouge A321 with 200 seats would have a lower CASM than a Jazz CRJ, but if you can only sell 50 seats once a day, that route would be better served by the RJ.
CASM isn't a fixed value for an aircraft either, flying a jet under 300nm isn't very cost effective and widebody jets only seem to come out ahead (on CASM) of narrow bodies beyond 2000-3000nm. Bigger is usually better, and smaller narrow bodies have become cost effective on longer thin, less competitive routes, or those that aren't overly price sensitive. For AC with the EMJ that translates to YYZ-SEA, YYZ-PDX, YYZ-YQR, YYZ-DEN, YYZ-YOW (where frequency is important).
While there may be thought that has been given to future staff shortages at the regionals, the main reason the Regional Replacement Aircraft is going to be implemented is because many regional routes have grown to the point where they would be more cost effectively served by a larger aircraft. For example, YYZ-BNA (Nashville) is now flown 3 times a day on RJ's, instead of adding a forth RJ per day, why not service the route using 3 A319's daily?
Fuel and crew costs are the obvious factors associated with a routes CASM, other factors include the cost of having a crew overnight and paid expenses versus starting at home base early and conducting a turn, the complexity of having multiple types service one route, the availability of gates has become an issue at YYZ and YUL, not to mention ATC costs, and landing fees.