In my opinion the "Manhole Fire"-Mission is a little underrated compared with other missions that require 2 fire engines.
To example I checked out my last alarms:
Garden shed fire: ~ 706 Credits
Burning truck: ~ 1172 Credits
Caravan ire: ~ 1161 Credits
Room fire: ~ 1680 Credits
Little field fire: ~ 1450 Credits
Manhole fire: ~ 233 Credits