Total cost of running the department for a year, divided by the number of callouts. Possibly weight the callout types depending on approximate resources consumed.
Though I am fully in support of just letting the house burn.
These costs still exist in a hypothetical year with zero fires, though. Additionally, people whose house were just on fire with zero insurance are probably not in a good financial position (anymore), and they are unlikely to be able to pay…
If you don't have the money, then a lien is placed on the property and it will be sold to pay the debt. Or you can take out a mortgage to buy it. In any case you'll be richer than if the house is burned down.
Ability to pay still not guaranteed, depending on the financial situation. Family can easily have more that's than assets to their name, preventing collection.
Though I am fully in support of just letting the house burn.