Answer by Amritansh Chaturvedi:
NaMo's basic requirements are taken care of by the Central Government. Hence he spends his salary only on his personal day-to-day requirements and saves the rest.
He had donated 21 lakhs from his personal savings (money he had saved from his Chief Minister's salary in 10 years) for educating Gujarat government staffs' kids, mainly daughters of drivers and peons working for the Gujarat Government).
He might do something similar in future with the money he saves from his income as the Prime Minister.