I bet this have been brought up before but here is my take on how to add custom calls in the game.
-First do not let the player decide how much money each mission earns, have it based it off of the number of apparatus required for each call. So maybe each fire engine is $500 and each PD car is $250 and each Heavy Rescue Truck is $750. This way people do not make a custom mission that is 1 Fire Truck for $100,000. This way the missions are set up fairly
-Second, the time a mission takes to complete is also based off the apparatus where each apparatus required adds an extra minute for time mission takes to complete.(Will not count for EMS)
In the end if someone wants to have a mission with just 1 Engine then it will be a minute long and they will make 500, but if they make a mission that require 25 vehicles to the scene then they will make around 10k-20k but it will take much of their resources and 25 mins.
Let me know what you guys think.