As far as i can see, you could potentially earn say 1000 kumi for a B rank or C rank mission if the kage sets the reward as such. Imo i think I would set certain missions with higher priorities than others as a reason to increase the reward.
Eg: these bandits have been raiding this village they need to be dealth with now. High priority. B rabk.
Please escort our trader to a neighbkuring village. Priority medium. B ranj.
High priority could be worth 1k and medium as 750 etc