In case they prove themselves to be active, not being stingy with the reward serves as appeal of a good noble too.
Because this time it is a monster subjugation, the reward is adjusted proportionally by contract to the number of monsters they defeat.
Although I am not sure, it must also be connected to the feeling of security that there won’t be a situation where they are told that there isn’t enough reward money after they they defeated too many monsters.
“For an Associate Baron 500 troops are quite a lot.”