The board of county commissioners shall set aside from any funds in the county treasury not specifically appropriated or set aside for any other purpose the following sum of money to be used by the state's attorney of the county as a contingent fund for the purpose of defraying the necessary expenses that are not otherwise provided for in securing evidence, investigating criminal cases, and furthering justice:
1. Not less than five hundred dollars and not more than one thousand dollars in counties having a population of ten thousand inhabitants or less.2. Not less than one thousand dollars and not more than one thousand five hundred dollars in counties having a population of more than ten thousand and not more than twenty thousand inhabitants.3. Not less than one thousand five hundred dollars and not more than two thousand dollars in counties having a population of more than twenty thousand inhabitants.