assuming the price for the route is the same, the A318 will have higher costs such as much higher fuel usage and leasing price. Those are usually the biggest factors. You can compare the expenses on the details page of the flight.
every plane burns fuel in (mainly) two ways. The first is fuel per cycle. This is the fuel needed to taxi, take off and climb to cruise altitude. The second is fuel used per kilometer to cover the distance between two airports. Some planes have a high consumption of fuel per cycle. That makes them less profitable on short routes.
The A318 burns 2360 liter just to take off, and then another 1335 liter to reach Dubai.
The Dash 400 only burns 650 liter to take off, and then another 630 liter to reach Dubai.
In your example, fuel per passenger (over the flown distance) is more or less the same for both planes. But... the Airbus needs 18 liter to get one passenger in the air (with standard seats), while the Dash only needs 8 liter to take one passenger in the air.
And on short flights, maintenance on the Airbus is also relatively more expensive.
Both the smallest member of the 737NG family (737-600) and the smallest member of the A320-family (A318-100) are not god moneymakers. for 120 seats planes try the Embrear 195 ;)
Both the smallest member of the 737NG family (737-600) and the smallest member of the A320-family (A318-100) are not god moneymakers. for 120 seats planes try the Embrear 195 ;)
Boeing 737-600, 700 and Airbus 318, 319 are good in making profit for slightly longer short haul flights with not very high demand. :)